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

[Aayush Shrivastava via cfe-commits]

iamaayushrivastava wrote:

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

Done. I've updated the commit description to explain the root cause and fix, and added a release note under "Bug Fixes to C++ Support" referencing #GH198890.

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