Avoid string_as_array

[tamird]

This function is part of the msan-unclean chain described in
protocolbuffers/protobuf#1099.

We see this on the go1.6 branch in the main repo:

==1953==WARNING: MemorySanitizer: use-of-uninitialized-value
    #0 0x1a8be89 in std::string::size() const /usr/lib/gcc/x86_64-linux-gnu/4.9/../../../../include/c++/4.9/bits/basic_string.h:725:26
    #1 0x1a8be89 in std::string::empty() const /usr/lib/gcc/x86_64-linux-gnu/4.9/../../../../include/c++/4.9/bits/basic_string.h:822
    #2 0x1a8be89 in google::protobuf::string_as_array(std::string*) /go/src/github.com/cockroachdb/c-protobuf/internal/src/google/protobuf/stubs/stl_util.h:85
    #3 0x1a8be89 in google::protobuf::io::mutable_string_data(std::string*) /go/src/github.com/cockroachdb/c-protobuf/internal/src/google/protobuf/io/zero_copy_stream_impl_lite.h:361

Defining LANG_CXX11 morphs this into:

==12370==WARNING: MemorySanitizer: use-of-uninitialized-value
    #0 0x1533d61 in std::string::_Rep::_M_is_leaked() const /usr/lib/gcc/x86_64-linux-gnu/4.9/../../../../include/c++/4.9/bits/basic_string.h:192:24
    #1 0x1533d61 in std::string::_M_leak() /usr/lib/gcc/x86_64-linux-gnu/4.9/../../../../include/c++/4.9/bits/basic_string.h:316
    #2 0x1533d61 in std::string::operator[](unsigned long) /usr/lib/gcc/x86_64-linux-gnu/4.9/../../../../include/c++/4.9/bits/basic_string.h:860
    #3 0x1533d61 in google::protobuf::io::mutable_string_data(std::string*) /go/src/github.com/cockroachdb/c-protobuf/internal/src/google/protobuf/io/zero_copy_stream_impl_lite.h:363

But this code path is also hit by other msan-unclean chains, so we
end up with fewer blacklisted functions overall.

The string_as_array callsite affected by this:
https://github.com/google/protobuf/blob/v3.0.0-beta-2/src/google/protobuf/io/zero_copy_stream_impl_lite.h#L359:L367

[tamird]

Hm, turns out I have to suppress std::string::size() anyway, so this isn't as compelling as I thought.

[petermattis]

Probably good to indicate we have a C++11 compiler, though.

LGTM