[clang] [clang] Fix crash in VisitVarTemplatePartialSpecializationDecl on failed instantiation (PR #200161)

[Matheus Izvekov via cfe-commits]

mizvekov wrote:

Wait, never mind, please roll back the last commit. On my previous comment I missed that the reason you added the Invalid flag was because it was checking the error before the declaration was created, so we couldn't just call setInvalidDecl just yet.

The last change made things worse because `SubstTSI` is just doing the same role as the Invalid flag in a roundabout way.

https://github.com/llvm/llvm-project/pull/200161