[clang] [clang] Bail out if the result of function template instantiation is not a function type. (PR #69459)

[Shafik Yaghmour via cfe-commits]

shafik wrote:

Please next time add a detailed description to your PR. This is key for reviewers to fully understand at a glance what the fix is for.

This is also key for users of git log to be able to understand changes at a glance without having to dig in. Folks often use git log to track down recent changes to explain changes in behavior.

https://github.com/llvm/llvm-project/pull/69459