[clang] [Clang] Enable -Wunused-template under -Wall (PR #206123)

[Erich Keane via cfe-commits]

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

I think I'm ok with this, we should make sure it is CLEAR to folks that they can disable this with `-Wno-unused-template` (so they don't have to throw out all of `-Wunused` if they want to suppress just this), so hopefully the Release Notes are sufficient here.  I suspect we'll get quite a few angry folks about this one though :D 

https://github.com/llvm/llvm-project/pull/206123