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

[Eric Astor via cfe-commits]

ericastor wrote:

> 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!).

This is a fair point... how are standard statement attributes handled inside of templates? (Assuming clang supports that.)

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