[clang] [llvm] [clang-tools-extra] [Clang][Sema] Don't say "is declared here" for invalid template locations (PR #71264)

[Justin Bogner via cfe-commits]

bogner wrote:

> That's a lot of repeated code. Should we have a "NoteTemplateLocation" function(s) of some sort that would do the check and the diag as appropriate?

Makes sense to me, and also makes @AaronBallman's idea of emitting notes referring to the external source a little easier. Done in the latest update.

https://github.com/llvm/llvm-project/pull/71264