[llvm] [X86] Correctly call 16 byte atomic helpers on x86_64 Windows (PR #181356)

[Eli Friedman via llvm-commits]

https://github.com/efriedma-quic commented:

There's some work going on to make actual ABI lowering available at the IR level, but that's not usable yet.  CC @nikic 

llvm/include/llvm/Analysis/TargetLibraryInfo.h has some ABI-related helpers.  Adding something there would be helpful for anyone trying to track down all our IR-level ABI assumptions.

https://github.com/llvm/llvm-project/pull/181356