This matches the HIR changes in rust-lang#134228, which introduced `PatExpr` to hold the
subset of "expressions" that can appear in a pattern.

…-errors

mir_build: Clarify some code for lowering `hir::PatExpr` to THIR

A few loosely-related improvements to the code that lowers certain parts of HIR patterns to THIR.

I was originally deferring this until after rust-lang#136529, but that PR probably won't happen, whereas these changes should hopefully be uncontroversial.

r? Nadrieril or reroll

Rollup of 11 pull requests

Successful merges:

 - rust-lang#133312 (triagebot: automatically add more rustdoc related labels)
 - rust-lang#134016 (Stabilize `const_is_char_boundary` and `const_str_split_at`.)
 - rust-lang#135813 (CI: split i686-mingw job to three free runners)
 - rust-lang#136879 (Add safe new() to NotAllOnes)
 - rust-lang#136971 (Add a new check-pass UI test for returning `impl Fn(T) -> impl Trait`)
 - rust-lang#136983 (Prepare standard library for Rust 2024 migration)
 - rust-lang#137002 (Fix early lint check desc in query)
 - rust-lang#137006 (borrowck diagnostics cleanup: remove an unused and a barely-used field)
 - rust-lang#137026 (Stabilize (and const-stabilize) `integer_sign_cast`)
 - rust-lang#137028 (mir_build: Clarify some code for lowering `hir::PatExpr` to THIR)
 - rust-lang#137032 (Decode metadata buffer in one go)

r? `@ghost`
`@rustbot` modify labels: rollup

Rollup merge of rust-lang#137028 - Zalathar:thir-pat-expr, r=compiler-errors

mir_build: Clarify some code for lowering `hir::PatExpr` to THIR

A few loosely-related improvements to the code that lowers certain parts of HIR patterns to THIR.

I was originally deferring this until after rust-lang#136529, but that PR probably won't happen, whereas these changes should hopefully be uncontroversial.

r? Nadrieril or reroll