[clang] [Sema] Fix crash in Sema::FindInstantiatedDecl (PR #96509)

[Vlad Serebrennikov via cfe-commits]

https://github.com/Endilll commented:

Makes sense to me, but wait for other reviewers.
You published this PR on the first day of WG21 meeting, so it didn't get the usual amount of attention.

https://github.com/llvm/llvm-project/pull/96509