[clang] [clang][CodeGen][MSVC] Match how MSVC returns vector types from member functions (PR #157365)

[Henry Baba-Weiss via cfe-commits]

henrybw wrote:

> Thanks for researching the `__m64` case!

Thanks for the reviews! I have updated the PR to handle the `__vectorcall` `__m64` case correctly, and added test cases for 64-bit vectors on x86, x86-64, and ARM64. I have also reworded the commit message and the release note slightly to reflect the updated changes.

Looks like I'll also need to rebase this before landing (my changes to the Clang ABI versions header have a merge conflict), but I wanted to get this latest iteration out for review first before rebasing, so I don't blow away the context of the existing review comments.

https://github.com/llvm/llvm-project/pull/157365