[ASan] Added __cplusplus guard around the C++ code so it is safe to include...

[ASan] Added __cplusplus guard around the C++ code so it is safe to include asan_mapping.h in assembly files.

This will avoid addind asan_shadow_defines.h in D114558.

Reviewed By: vitalybuka

Differential Revision: https://reviews.llvm.org/D115348