[clang] Reland "[Clang] Enable -Wunused-template under -Wall" (PR #208001)

[Vassil Vassilev via cfe-commits]

vgvassilev wrote:

I’d think the diagnostic is valid here. If that’s unused we can either delete that code of mark it with the maybe_unused attribute. 

https://github.com/llvm/llvm-project/pull/208001