[clang] [clang][ASTImporter] Fix possible crash at import of function template (PR #124273)

[Balázs Kéri via cfe-commits]

balazske wrote:

It seems that this test is not reducible any further (except very small changes) (to get the original crash). I could remove only few lines.

https://github.com/llvm/llvm-project/pull/124273