[instrinsics] Add @llvm.memcpy.inline instrinsics

Summary:
This is a follow up on D61634. It adds an LLVM IR intrinsic to allow better implementation of memcpy from C++.
A follow up CL will add the intrinsics in Clang.

Reviewers: courbet, theraven, t.p.northover, jdoerfert, tejohnson

Subscribers: hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D71710