[Builtins] Teach Clang about memccpy

Summary: Hopefully, -fno-builtin-memccpy will work now. Required for https://reviews.llvm.org/D67986.

Reviewers: aaron.ballman, rsmith

Reviewed By: aaron.ballman

Subscribers: RKSimon, kristina, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D68377