HIR: separate AST and HIR lints

[nrc]

Lints are a pain at the moment because they operate on the AST but need type info which belongs to the HIR. We should have two kinds of lints - purely syntactic ones which operate on the (expanded) AST and run early (before AST->HIR lowering) and ones which can access type info and operate on the HIR (in the current position). We probably need to think about how this will interact with the MIR too.

[Manishearth]

#28192 moves lints to HIR so that existing lints can upgrade easily. As a next step we can move that folder to librustc_front/lint and restore the original librustc_lint. The only builtin lint that needs the AST is the unused parentheses one, and at the moment ExprParen exists in the HIR so it's not yet broken.

[eddyb]

@Manishearth I'm not sure it's just the parenthesis lint, but at least that one can be moved to an AST lint later, if/when ExprParen is gone from the HIR.

[nrc]

Closed by #28349