[llvm] [Transforms] Introduce BuildBuiltins.h atomic helpers (PR #134455)

[Michael Kruse via llvm-commits]

Meinersbur wrote:

@jyknight Here the proof that emitting sized libcall atomics early is better than letting AtomicExpandPass do it: https://godbolt.org/z/bGW37TMYh. In this case it skips tail call elimination.

https://github.com/llvm/llvm-project/pull/134455