Abstract AST lowering resolver
This PR adds new methods for `ResolverAstLoweringExt` for future use in rust-lang/rust#153489 and abstracts resolver in lowering through generic parameters.
> ~This PR splits resolver for AST lowering into two parts: mutable and readonly. This will allow us to use borrowed resolver in rust-lang/rust#153489, when we will not steal but borrow resolver from resolve stage.~
Second step for rust-lang/rust#153489.
r? @petrochenkov
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