[clang] [Clang] Fix finding instantiated decls for class template specializations during instantiation (PR #72346)

[Erich Keane via cfe-commits]

https://github.com/erichkeane commented:

I don't have a great feel if this is the right fix, but if it doesn't break anything in the tests, and does fix something, this is likely acceptable for now.

This DOES need a release note, and as Shafik says: this should likely be placed in an existing/related test file in a namespace GH70735.

https://github.com/llvm/llvm-project/pull/72346