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

[Zahira Ammarguellat via cfe-commits]

zahiraam wrote:

> If we want ` __attribute__((packed))` to ignore "required" alignment, we can do that; there's no MSVC compatibility involved because MSVC doesn't support the attribute form. But if we want to do that, we should make it universal. And in particular, there's no reason to do it for vectors, but not x86_fp80.

Done.

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