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

[Oliver Hunt via cfe-commits]

ojhunt wrote:

Something I just discovered: `-mlong-double-80` explicitly overrides the long double alignment, even on win32, from 8 bytes to 16 bytes - even though long double is fp80 their, so it's doing something questionable with the declared natural alignment.

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