[clang] [clang][X86] Return Win64 __int128 indirectly (PR #210358)

[Daniel Paoliello via cfe-commits]

dpaoliello wrote:

Correct me if I'm wrong, but this seems like a breaking change and would need release notes (at a minimum).

Also, what's LLVM's behavior with returning a 128bit struct? I want to make sure that `i128` and `stuct { i64; i64}` are handled the same, since folks love to plan games like with public header vs implementation.

https://github.com/llvm/llvm-project/pull/210358