[clang] Fix to attribute plugins reaching an unreachable (PR #70877)

[Aaron Ballman via cfe-commits]

AaronBallman wrote:

Oh wow, same failure a second time...

Ohh, this looks familiar now, but... it was already fixed, so why are we still hitting this failure?
```
| expected: clang::TreeTransform<(anonymous namespace)::CurrentInstantiationRebuilder>::RebuildArrayType(clang::QualType, clang::ArraySizeModifier, llvm::APInt const*, clang::Expr*, unsigned int, clang::SourceRange)
| got: 0,   clang::TreeTransform<(anonymous namespace)::CurrentInstantiationRebuilder>::RebuildArrayType(clang::QualType, clang::ArrayType::ArraySizeModifier, llvm::APInt const*, clang::Expr*, unsigned int, clang::SourceRange)
```
should have been fixed by https://github.com/llvm/llvm-project/commit/f5f4c5b313fc809b9b1b9d600d0ec3cec0d12941 over two weeks ago. CC @Endilll 

https://github.com/llvm/llvm-project/pull/70877