Auto merge of #110100 - compiler-errors:no-infer-pred-must-hold, r=jackh726

do not allow inference in `predicate_must_hold` (alternative approach)

See the FCP description for more info, but tl;dr is that we should not return `EvaluatedToOkModuloRegions` if an obligation may hold only with some choice of inference vars being constrained.

Attempts to solve this in the approach laid out by lcnr here: rust-lang/rust#109558 (comment), rather than by eagerly replacing infer vars with placeholders which is a bit too restrictive.

r? `@ghost`

Author: bors