libstdc++: Make Asan detection work for Clang [PR103453]

Clang doesn't define __SANITIZE_ADDRESS__ so use its __has_feature check
to detect Asan instead.

libstdc++-v3/ChangeLog:

	PR libstdc++/103453
	* config/allocator/malloc_allocator_base.h
	(_GLIBCXX_SANITIZE_STD_ALLOCATOR): Define for Clang.
	* config/allocator/new_allocator_base.h
	(_GLIBCXX_SANITIZE_STD_ALLOCATOR): Likewise.
2 files changed