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

[Zahira Ammarguellat via cfe-commits]

zahiraam wrote:

The `libcxx` fails are unrelated to this PR and have seen in some other PRs. 

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