[libc++] Use builtins when redeclaring <string.h> functions

When we define the const-correct overloads of <string.h> functions in
libc++ itself, use builtins whenever possible. This avoids depending on
the presence of these functions in the C library headers.

Also, as a fly-by, improve the tests for these functions since we
basically didn't check anything but their signature. We could have
used the wrong builtin (as long as the signature matched) without ever
noticing, which was quite scary.

Differential Revision: https://reviews.llvm.org/D138684