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

[Matheus Izvekov via cfe-commits]

https://github.com/mizvekov commented:

Also don't forget to update the commit description, and also add a release note.

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