[libc] [llvm] [libc] Change __builtin_memcpy to inline_memcpy. (PR #158345)

[Michael Jones via llvm-commits]

https://github.com/michaelrj-google approved this pull request.

LGTM. 

For context the reason behind this change is MSVC doesn't support `__builtin_memcpy` so we're moving everything to `inline_memcpy`. We can discuss how to adjust this so it's more optimal later but this change is needed to unblock MSVC.

https://github.com/llvm/llvm-project/pull/158345