[TargetLibraryInfo] memcpy and memset have optimized codegen.

Summary:
Add memcpy and memset to the list of function with optimized codegen.
hasOptimizedCodeGen() was returning false for them.
This was not an issue when working with c++ as clang typically emits an
intrinsic rather than a libfunc call, but when working with IR,
this would cause some issues with sanitizers which would not correctly
mark the function as nobuiltin in `maybeMarkSanitizerLibraryCallNoBuiltin()`.

Subscribers: llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D78991