[clang] Enable unguarded availability diagnostic on instantiated template functions (PR #91699)

[Cooper Partin via cfe-commits]

https://github.com/coopp approved this pull request.

Looks fine to me.  I assume this doesn't impact other test passes right?  Now there is work being done when before it was doing an early out for template instantiations.

https://github.com/llvm/llvm-project/pull/91699