Rollup merge of #104296 - compiler-errors:opaque-ty-ffi-normalization-cycle, r=lcnr

Walk types more carefully in `ProhibitOpaqueTypes` visitor

The visitor didn't account for the case where you could have `<TAIT as Trait>::Assoc` normalize to itself, in the case of a `type TAIT = impl Trait` with an unconstrained associated type. That causes the visitor to loop on the same type over and over.

Fixes #104291

diff --cc compiler/rustc_lint/src/types.rs
Simple merge