or-patterns: Push PatKind/PatternKind::Or at top level to HIR & HAIR

[Centril]

Following up on work in #64111, #63693, and #61708, in this PR:

• We change hair::Arm.patterns: Vec<Pattern<'_>> into hir::Arm.pattern: Pattern<'_>.

  • fn hair::Arm::top_pats_hack is introduced as a temporary crutch in MIR building to avoid more changes.

• We change hir::Arm.pats: HirVec<P<Pat>> into hir::Arm.pat: P<Pat>.

  • The hacks in rustc::hir::lowering are removed since the representation hack is no longer necessary.

  • In some places, fn hir::Arm::top_pats_hack is introduced to leave some things as future work.

  • Misc changes: HIR pretty printing is adjusted to behave uniformly wrt. top/inner levels, rvalue promotion is adjusted, regionck, and dead_code is also.

  • Type checking is adjusted to uniformly handle or-patterns at top/inner levels.
    To make things compile, p_0 | ... | p_n is redefined as a "reference pattern" in fn is_non_ref_pat for now. This is done so that reference types are not eagerly stripped from the expected: Ty<'tcx>.

  • Liveness is adjusted wrt. the unused_variables and unused_assignments lints to handle top/inner levels uniformly and the handling of fn parameters, let locals, and match arms are unified in this respect. This is not tested for now as exhaustiveness checks are reachable and will ICE.

  • In check_match, checking @ and by-move bindings is adjusted. However, exhaustiveness checking is not adjusted the moment and is handled by @dlrobertson in WIP: Initial implementation of or-pattern handling in MIR #63688.

  • AST borrowck (construct.rs) is not adjusted as AST borrowck will be removed soon.

r? @matthewjasper
cc @dlrobertson @varkor @oli-obk