Auto merge of #120497 - compiler-errors:modulize, r=lcnr

Move predicate, region, and const stuff into their own modules in middle

This PR mostly moves things around, and in a few cases adds some `ty::` to the beginning of names to avoid one-off imports.

I don't mean this to be the most *thorough* move/refactor. I just generally wanted to begin to split up `ty/mod.rs` and `ty/sty.rs` which are huge and hard to distinguish, and have a lot of non-ty stuff in them.

r? lcnr

compiler/rustc_middle/src/ty/consts.rs

@@ -7,6 +7,7 @@ use rustc_hir as hir;
 use rustc_hir::def::{DefKind, Res};
 use rustc_hir::def_id::LocalDefId;
 use rustc_macros::HashStable;
+use rustc_type_ir::ConstKind as IrConstKind;
 use rustc_type_ir::{ConstTy, IntoKind, TypeFlags, WithCachedTypeInfo};
 
 mod int;
@@ -19,7 +20,7 @@ use rustc_span::Span;
 use rustc_span::DUMMY_SP;
 pub use valtree::*;
 
-use super::sty::ConstKind;
+pub type ConstKind<'tcx> = IrConstKind<TyCtxt<'tcx>>;
 
 /// Use this rather than `ConstData`, whenever possible.
 #[derive(Copy, Clone, PartialEq, Eq, PartialOrd, Ord, Hash, HashStable)]