[clang] [clang] Fix vectorcall HVA/HFA returns on x86 MSVC ABI (PR #203925)

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kin4stat wrote:

Hi! This seems to overlap with [llvm/llvm-project#113104 · [clang][CodeGen] Fix MSVC ABI…](https://github.com/llvm/llvm-project/pull/113104), which fixes the same issue (#63417) and has been updated based on review feedback.

A couple of differences I noticed:

Under vectorcall, MSVC also returns HVAs in vector registers from instance methods. Here isTrivialForABI is set before the FI.isInstanceMethod() check, so instance methods still return indirectly.
The homogeneous-aggregates.cpp part uses an x86_64-windows-gnu triple (Itanium C++ ABI), so it doesn’t go through MicrosoftCXXABI and passes with or without the fix.

Would you be OK with continuing in [llvm/llvm-project#113104 · [clang][CodeGen] Fix MSVC ABI…](https://github.com/llvm/llvm-project/pull/113104)? Reviews there are very welcome.

https://github.com/llvm/llvm-project/pull/203925