[clang] [clang] Represent array refs as `TemplateArgument::Declaration` (PR #80050)

[Andrey Ali Khan Bolshakov via cfe-commits]

bolshakov-a wrote:

I think the problem is that the generalized NTTP feature is still raw. It should not break already working features, namely array reference NTTPs, hence they are swithed back to the old `TemplateArgument::Declaration`, as they worked prior to #78041, from the new `StructuralValue`.

https://github.com/llvm/llvm-project/pull/80050