Rollup merge of #133493 - lcnr:fulfill-fudge, r=compiler-errors

do not constrain infer vars in `find_best_leaf_obligation`

This ended up causing an ICE by making the following code path reachable by incorrectly constraining an inference variable while computing the best obligation for a preceding ambiguity. Closes #129444.

https://github.com/rust-lang/rust/blob/f2abf827c128120ed7a874d02973947968c158b8/compiler/rustc_trait_selection/src/solve/fulfill.rs#L312-L314

I have to be honest, I don't fully understand how that change removes all the additional diagnostics :3

r? `@compiler-errors`

Author: matthiaskrgr