Add Alignment argument to IRBuilder CreateAtomicRMW and CreateAtomicCmpXchg.

Add Alignment argument to IRBuilder CreateAtomicRMW and CreateAtomicCmpXchg.

And then push those change throughout LLVM.

Keep the old signature in Clang's CGBuilder for now -- that will be
updated in a follow-on patch (D97224).

The MLIR LLVM-IR dialect is not updated to support the new alignment
attribute, but preserves its existing behavior.

Differential Revision: https://reviews.llvm.org/D97223