[clang] Fix a regression with alignas on structure members in C (PR #98642)

[Aaron Ballman via cfe-commits]

AaronBallman wrote:

> The part I'm confused about is that this commit doesn't appear to simply be adding "or `alignas`" to some code that said "if `_Alignas`". Yes, the newly added `isAlignas()` checks do cover both attributes, but `_Alignas` was still working even after [b9cf7f1](https://github.com/llvm/llvm-project/commit/b9cf7f1066cc7ec11c6074d10b0f2ec9a3ae72d9).
> 
> That is, in clang trunk, this test does still pass when parsed as -std=c23:
> 
> ```
> struct X {
>     _Alignas(8) char n;
> };
> 
> static_assert(_Alignof(struct X) == 8, "");
> ```
> 
> So why does this PR need to add new code to move `alignas`, instead of having `alignas` use whatever codepath was already handling `_Alignas`? Or, alternatively remove whatever code was formerly handling `_Alignas`, in favor of this new code?
> 
> Or, are `alignas` and `_Alignas` actually getting parsed differently, even still, so only `alignas` needs to be moved?

Ah, thank you for the further details! The issue boils down to the fact that `alignas` is not an attribute in C23 (it's a type-specifier-qualifier) but it is an attribute in C++. So we hit awkward situations where we claim the attribute is a C++11-style attribute, such as in `ProcessDeclAttribute()`. We explicitly disallow C++11 attributes on the declaration specifier:
```
  // Apply decl attributes from the DeclSpec if present.
  if (!PD.getDeclSpec().getAttributes().empty()) {
    ProcessDeclAttributeList(S, D, PD.getDeclSpec().getAttributes(),
                             ProcessDeclAttributeOptions()
                                 .WithIncludeCXX11Attributes(false)
                                 .WithIgnoreTypeAttributes(true));
  }
```
but when the attribute is on the declaration itself, we allow C++11 attributes:
```
ProcessDeclAttributeList(S, D, NonSlidingAttrs);
```

So there *is* a better way to address this and I think it actually fixes a bug that I didn't notice until now with non-structure use. The changes look a bit odd because I also had to make an adjustment to ensure that `matrix_type` continues to work (it's an odd beast that already required special handling, that special handling is now needed in two places).

https://github.com/llvm/llvm-project/pull/98642