clang older than version 9 fails to build Throttler.h

[bneradt]

Ubuntu clang CI builds currently fail when compiling Throttler.h. This is a new file that was introduced with the following PR: #7279.

Output is currently available from CI here:
https://ci.trafficserver.apache.org/job/ubuntu_18_04-master/compiler=clang,label=ubuntu_18_04,type=release/1041/console

It complains about the use of atomic and chrono in Throttler.h:

In file included from ../../../lib/records/RecDebug.cc:24:
In file included from ../../../lib/records/P_RecUtils.h:26:
In file included from ../../../include/tscore/Diags.h:38:
In file included from ../../../include/tscore/LogMessage.h:28:
In file included from ../../../include/tscore/Throttler.h:26:
/usr/bin/../lib/gcc/x86_64-linux-gnu/7.5.0/../../../../include/c++/7.5.0/atomic:194:7: error: exception specification of explicitly defaulted default constructor does not match the calculated one
atomic() noexcept = default;
^
../../../include/tscore/Throttler.h:117:26: note: in instantiation of template class 'std::atomic<std::chrono::time_point<std::chrono::_V2::system_clock, std::chrono::duration<long, std::ratio<1, 1000000000> > > >' requested here
std::atomic _last_allowed_time{TimePoint{}};
^
1 error generated.
Makefile:951: recipe for target 'RecDebug.o' failed
make[2]: *** [RecDebug.o] Error 1
make[2]: *** Waiting for unfinished jobs....
In file included from ../../../lib/records/RecFile.cc:28:
In file included from ../../../lib/records/P_RecUtils.h:26:
In file included from ../../../include/tscore/Diags.h:38:
In file included from ../../../include/tscore/LogMessage.h:28:
In file included from ../../../include/tscore/Throttler.h:26:
/usr/bin/../lib/gcc/x86_64-linux-gnu/7.5.0/../../../../include/c++/7.5.0/atomic:194:7: error: exception specification of explicitly defaulted default constructor does not match the calculated one
atomic() noexcept = default;
^
../../../include/tscore/Throttler.h:117:26: note: in instantiation of template class 'std::atomic<std::chrono::time_point<std::chrono::_V2::system_clock, std::chrono::duration<long, std::ratio<1, 1000000000> > > >' requested here
std::atomic _last_allowed_time{TimePoint{}};
^
1 error generated.
Makefile:951: recipe for target 'RecFile.o' failed
make[2]: *** [RecFile.o] Error 1
In file included from ../../../lib/records/RecConfigParse.cc:34:
In file included from ../../../lib/records/P_RecUtils.h:26:
In file included from ../../../include/tscore/Diags.h:38:
In file included from ../../../include/tscore/LogMessage.h:28:
In file included from ../../../include/tscore/Throttler.h:26:
/usr/bin/../lib/gcc/x86_64-linux-gnu/7.5.0/../../../../include/c++/7.5.0/atomic:194:7: error: exception specification of explicitly defaulted default constructor does not match the calculated one
atomic() noexcept = default;
^
../../../include/tscore/Throttler.h:117:26: note: in instantiation of template class 'std::atomic<std::chrono::time_point<std::chrono::_V2::system_clock, std::chrono::duration<long, std::ratio<1, 1000000000> > > >' requested here
std::atomic _last_allowed_time{TimePoint{}};
^
1 error generated.
Makefile:951: recipe for target 'RecConfigParse.o' failed
make[2]: *** [RecConfigParse.o] Error 1
In file included from ../../../lib/records/P_RecCore.cc:33:
In file included from ../../../lib/records/P_RecUtils.h:26:
In file included from ../../../include/tscore/Diags.h:38:
In file included from ../../../include/tscore/LogMessage.h:28:
In file included from ../../../include/tscore/Throttler.h:26:
/usr/bin/../lib/gcc/x86_64-linux-gnu/7.5.0/../../../../include/c++/7.5.0/atomic:194:7: error: exception specification of explicitly defaulted default constructor does not match the calculated one
atomic() noexcept = default;
^
../../../include/tscore/Throttler.h:117:26: note: in instantiation of template class 'std::atomic<std::chrono::time_point<std::chrono::_V2::system_clock, std::chrono::duration<long, std::ratio<1, 1000000000> > > >' requested here
std::atomic _last_allowed_time{TimePoint{}};
^
1 error generated.
Makefile:951: recipe for target 'P_RecCore.o' failed
make[2]: *** [P_RecCore.o] Error 1
In file included from ../../../lib/records/RecHttp.cc:24:
In file included from ../../../lib/records/I_RecCore.h:28:
In file included from ../../../include/tscore/Diags.h:38:
In file included from ../../../include/tscore/LogMessage.h:28:
In file included from ../../../include/tscore/Throttler.h:26:
/usr/bin/../lib/gcc/x86_64-linux-gnu/7.5.0/../../../../include/c++/7.5.0/atomic:194:7: error: exception specification of explicitly defaulted default constructor does not match the calculated one
atomic() noexcept = default;
^
../../../include/tscore/Throttler.h:117:26: note: in instantiation of template class 'std::atomic<std::chrono::time_point<std::chrono::_V2::system_clock, std::chrono::duration<long, std::ratio<1, 1000000000> > > >' requested here
std::atomic _last_allowed_time{TimePoint{}};
^
1 error generated.
Makefile:951: recipe for target 'RecHttp.o' failed
make[2]: *** [RecHttp.o] Error 1
In file included from ../../../lib/records/RecCore.cc:29:
In file included from ../../../mgmt/RecordsConfig.h:26:
In file included from ../../../lib/records/P_RecCore.h:31:
In file included from ../../../lib/records/I_RecCore.h:28:
In file included from ../../../include/tscore/Diags.h:38:
In file included from ../../../include/tscore/LogMessage.h:28:
In file included from ../../../include/tscore/Throttler.h:26:
/usr/bin/../lib/gcc/x86_64-linux-gnu/7.5.0/../../../../include/c++/7.5.0/atomic:194:7: error: exception specification of explicitly defaulted default constructor does not match the calculated one
atomic() noexcept = default;
^
../../../include/tscore/Throttler.h:117:26: note: in instantiation of template class 'std::atomic<std::chrono::time_point<std::chrono::_V2::system_clock, std::chrono::duration<long, std::ratio<1, 1000000000> > > >' requested here
std::atomic _last_allowed_time{TimePoint{}};
^
1 error generated.
Makefile:951: recipe for target 'RecCore.o' failed
make[2]: *** [RecCore.o] Error 1
make[2]: Leaving directory '/var/jenkins/workspace/ubuntu_18_04-master/compiler/clang/label/ubuntu_18_04/type/release/build/BUILDS/lib/records'
Makefile:575: recipe for target 'all-recursive' failed