Rollup merge of #150688 - Zalathar:check-pat-lit, r=petrochenkov

typeck: Make it clearer that `check_pat_lit` only handles literal patterns

Nowadays, the `hir::PatExprKind` enum guarantees that “expressions” in patterns can only be paths or literals.

`PatExprKind::Path` is already handled by the previous match arm, so we can make this match arm explicitly match on `PatExprKind::Lit` without losing exhaustiveness.

There should be no actual change to compiler behaviour.

Author: JonathanBrouwer