[clang] [clang] deprecate alias, class templates without arg list after template kw (PR #94789)

[via cfe-commits]

cor3ntin wrote:

Ideally, i think the warning would only happen by default in pedantic modes, in C++23 and later (This was deprecated as part of P1787R6). It's a pretty noisy warning for an harmless construct.

However it would make sense than one should be able to opt-in the warning unconditionally. 

I'm not sure whether there is a good way to do that @AaronBallman 

I would also update most of our tests not to use `template` where it's deprecated, except in tests where we are explicitly testing that warning 

https://github.com/llvm/llvm-project/pull/94789