Auto merge of #118842 - Nadrieril:librarify-further, r=compiler-errors

Make exhaustiveness usable outside of rustc

With this PR, `rustc_pattern_analysis` compiles on stable (with the `stable` feature)! `rust-analyzer` will be able to use it to provide match-related diagnostics and refactors.

Two questions:
- Should I name the feature `nightly` instead of `rustc` for consistency with other crates? `rustc` makes more sense imo.
- `typed-arena` is an optional dependency but tidy made me add it to the allow-list anyway. Can I avoid that somehow?

r? `@compiler-errors`

compiler/rustc_mir_build/src/errors.rs

@@ -6,7 +6,7 @@ use rustc_errors::{
 };
 use rustc_macros::{Diagnostic, LintDiagnostic, Subdiagnostic};
 use rustc_middle::ty::{self, Ty};
-use rustc_pattern_analysis::{cx::MatchCheckCtxt, errors::Uncovered};
+use rustc_pattern_analysis::{errors::Uncovered, rustc::RustcMatchCheckCtxt};
 use rustc_span::symbol::Symbol;
 use rustc_span::Span;
 
@@ -454,7 +454,7 @@ pub enum UnusedUnsafeEnclosing {
 }
 
 pub(crate) struct NonExhaustivePatternsTypeNotEmpty<'p, 'tcx, 'm> {
-    pub cx: &'m MatchCheckCtxt<'p, 'tcx>,
+    pub cx: &'m RustcMatchCheckCtxt<'p, 'tcx>,
     pub expr_span: Span,
     pub span: Span,
     pub ty: Ty<'tcx>,