Enable unguarded availability diagnostic on instantiated template functions (#91699)

Availability diagnostic in instantiated template functions was
intentionally skipped in the original
[commit](https://github.com/llvm/llvm-project/commit/5cd57177a51abc7b0bfe18f70566572dbccab9a0)
years ago with a FIXME note.

I ran into this when working on diagnostics for HLSL. When I remove the
skip, it seems to be working just fine outputting expected messages. So,
unless I am missing something, I would keep it enabled and use it for
checking availability in HLSL templates as well.