[clang] Add support for __builtin_memcpy_inline

Summary: This is a follow up on D61634 and the last step to implement http://lists.llvm.org/pipermail/llvm-dev/2019-April/131973.html

Reviewers: efriedma, courbet, tejohnson

Subscribers: hiraditya, cfe-commits, llvm-commits, jdoerfert, t.p.northover

Tags: #clang, #llvm

Differential Revision: https://reviews.llvm.org/D73543