[libc++][NFC] Work around false positive ODR violations from ASan.

This works around a known issue in ASan. ASan doesn't instrument weak
symbols. Because instrumentation increases object size, the binary can
end up with two versions of the same object, one instrumented and one
not instrumented, with different sizes, which ASan will report as an ODR
violation. In libc++, this affects typeinfo for `std::bad_function_call`
which is emitted as a weak symbol in the test executable and as a strong
symbol in the shared library.

The main open issue for ASan appears to be
https://github.com/google/sanitizers/issues/1017.

Differential Revision: https://reviews.llvm.org/D119410