Auto merge of #119329 - Nadrieril:reveal-opaques-early, r=compiler-errors

Exhaustiveness: Statically enforce revealing of opaques

In rust-lang/rust#116821 it was decided that exhaustiveness should operate on the hidden type of an opaque type when relevant. This PR makes sure we consistently reveal opaques within exhaustiveness. This makes it possible to remove `reveal_opaque_ty` from the `TypeCx` trait which was an unfortunate implementation detail.

r? `@compiler-errors`

Author: bors