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

[Oliver Hunt via cfe-commits]

ojhunt wrote:

> MSVC always uses 64-bit `long double`. I don't know anything about attempts to use 80-bit `long double` on Windows except that they're asking for tons of complexity.

Right, that was what I thought (a few years ago I was _very_ jealous of win64 making that choice).

To understand what clang might be doing wrong, what happens if someone were to do something along the lines of (stealing from original failure) this?

```cpp
#include <array>                                                                        
struct Klass { some_overaligned_type_t field; };
                                                                                        
void test() {                                                                           
  std::array<Klass, 16> matrix;
  matrix.fill({}); 
} 
```

would that be expected to work?

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