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

[Folkert de Vries via cfe-commits]

folkertdev wrote:

> cc @folkertdev , how did Rust handle the f128 ABI passing change? Is Windows codgen just considered ABI unstable for long-tail types (i128, f128), or do you have some process for this?

The rust `f128` type is still unstable, so we're free to change its ABI (and will when we upgrade to LLVM 23). But `i128` is stable and so this might cause breakage, and I was not aware of GCC making this change. There is not much we can do though except follow suite, much like when the alignment of `i128` changed. 

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