[clang] [Clang][Windows] Preserve x86_fp80 and vector alignment under #pragma pack. (PR #208256)

[Eli Friedman via cfe-commits]

efriedma-quic wrote:

"aligned" attributes on typedefs are their own mess which is unrelated to this issue.  We should represent it as a type qualifier, but we don't, which causes various issues with template instantiation.

The reason `__m128` behaves the way it does on MSVC is that unlike LLVM, it isn't a builtin type; it's actually a struct with an `alignas` marking.

https://github.com/llvm/llvm-project/pull/208256