[libc++] Remove test suite workarounds on Apple with old Clangs

In 5fd17ab1, we worked around the Apple system headers not providing
const-correct overloads for some <string.h> functions. However, that
required an attribute that was only present in recent Clangs at the
time. We can now assume that all supported Clang versions on Apple
platforms do support that attribute.

Differential Revision: https://reviews.llvm.org/D100477