[Builtins] Treat `bcmp` as a builtin.

Summary:
This makes it consistent with `memcmp` and `__builtin_bcmp`.

Also see the discussion in https://reviews.llvm.org/D56593.

Reviewers: jyknight

Subscribers: kristina, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D58120

llvm-svn: 354023