Auto merge of #145993 - lcnr:allow-calling-opaques, r=BoxyUwU

`-Znext-solver` allow `ExprKind::Call` for not-yet defined opaques

Based on rust-lang/rust#146329. Revival of rust-lang/rust#140496. See the comment on `OpaqueTypesJank`. I've used the following document while working on this https://hackmd.io/Js61f8PRTcyaiyqS-fH9iQ.

Fixes rust-lang/trait-system-refactor-initiative#181. It does introduce one subtle footgun we may want to handle before stabilization, opened rust-lang/trait-system-refactor-initiative#230 for that. Also cc rust-lang/trait-system-refactor-initiative#231 for deref and index operations

r? `@BoxyUwU`

Author: bors