Rollup merge of #108319 - compiler-errors:dont-project-to-specializable-rpitits, r=lcnr

Don't project specializable RPITIT projection

This effective rejects specialization + RPITIT/AFIT (usages of `impl Trait` in traits) because the implementation is significantly complicated over making regular "default" trait method bodies work.

I have another PR that experimentally fixes all this, but the code may not be worth investing in.

Author: matthiaskrgr