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

[Eric Astor via cfe-commits]

ericastor wrote:

So... I think this turned out to be surprisingly easy. After all, we don't actually allow custom attributes to create entirely new attributes in the AST, since they can't create new legal AST entries... instead, the example creates AnnotateAttrs that can then be observed & acted on by the compiler-extending plugin.

It looks like reaching the same level of support just required adding AnnotateAttr support to the template instantiation logic. Am I missing anything else? @erichkeane

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