libasan: keep debuginfo on dlclose

[diorcety]

Currently if there is a leak in the code of a dynamic loaded library, which is already closed when the program's end, the report contains addresses without any information but "unknown module".
Seem's to be a known issue in many asan: google/sanitizers#89. but which is resolved in valgrind with the fix associated to https://bugs.kde.org/show_bug.cgi?id=79362

[syzop]

A feature like this would help us a lot.

Current situation

Backtraces for us look like this now:

Direct leak of 16 byte(s) in 1 object(s) allocated from:
    #0 0x560790cbdfb2 in calloc (/home/irc/unrealircd/bin/unrealircd+0x11cfb2)
    #1 0x560790e08539 in safe_alloc /home/irc/u6/src/support.c:715:6
    #2 0x7f72950c7ec9  (<unknown module>)
    #3 0x7f72950c5fdc  (<unknown module>)
    #4 0x7f72950c58c6  (<unknown module>)
    #5 0x560790d98b31 in _conf_set /home/irc/u6/src/conf.c:7840:13
    #6 0x560790d57221 in config_run_blocks /home/irc/u6/src/conf.c:2834:10

Stack frame 5, the _conf_set is code calling a linked list of "hooks" where modules can plug in. This particular hook is used by 39 different modules. Long story short: if I see a backtrace like this I really have no idea which module has a problem.

Not calling dlclose() is not an option

Someone mentioned in google/sanitizers#89 people should simply not call dlclose(). But then we would miss bugs/leaks that are otherwise detected. Like a module which has a global variable char *xyz that receives allocated memory, if we would skip calling dlclose() then it is no longer (detected as) a leak.