[libc++] Simplify when the sized global deallocations overloads are available (#114667)

There doesn't seem to be much benefit in always providing declarations
for the sized deallocations from C++14 onwards if the user explicitly
passed `-fno-sized-deallocation` to disable them. This patch simplifies
the declarations to be available exactly when the compiler expects sized
deallocation functions to be available.

GitOrigin-RevId: ef804d8f9b4ef4ff39e873d5910e94887519bdb6
1 file changed