[clang] [clang][frontend] Add support for attribute plugins for statement attributes (PR #110334)

[Erich Keane via cfe-commits]

https://github.com/erichkeane commented:

So the problem with statement attributes is that we don't really have a good way to do instantiation of them on a template, which is why we held off on this in the first place. The infrastructure for instantiation DOES happen on decl attributes automatically anyway, so that made it easier to do.

So please ensure there is some sort of hooks in the statement attributes to ensure that they get a chance to instantatiate themselves (and add tests with templates in them!).

https://github.com/llvm/llvm-project/pull/110334