[Clang][PP] Add the FILE_NAME builtin macro.

This patch adds the __FILE_NAME__ macro that expands to the
last component of the path, similar to __FILE__ except with
a guarantee that only the last path component (without the
separator) will be rendered.

I intend to follow through with discussion of this with WG14
as a potential inclusion in the C standard or failing that,
try to discuss this with GCC developers since this extension
is desired by GCC and Clang users/developers alike.

Differential Revision: https://reviews.llvm.org/D61756