[clang] [clang] require template arg list after template kw (PR #80801)

[Aaron Ballman via cfe-commits]

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

I think the changes are reasonable, but I'd like to wait a bit before landing to give @erichkeane a chance to review as templates code owner.

https://github.com/llvm/llvm-project/pull/80801