Rollup merge of #121695 - oli-obk:split_ty_utils, r=compiler-errors

Split rustc_type_ir to avoid rustc_ast from depending on it

unblocks #121576

and resolves a FIXME in `rustc_ast`'s `Cargo.toml`

The new crate is tiny, but it will get bigger in #121576

compiler/rustc_hir_analysis/src/check/errs.rs

@@ -1,9 +1,8 @@
 use rustc_hir as hir;
 use rustc_hir_pretty::qpath_to_string;
 use rustc_lint_defs::builtin::STATIC_MUT_REFS;
-use rustc_middle::ty::TyCtxt;
+use rustc_middle::ty::{Mutability, TyCtxt};
 use rustc_span::Span;
-use rustc_type_ir::Mutability;
 
 use crate::errors;