Address more review comments

- Add back various diagnostic methods on `Session`.

  It seems unfortunate to duplicate these in so many places, but in the
  meantime, making the API inconsistent between `Session` and `Diagnostic`
  also seems unfortunate.

- Add back TyCtxtAt methods

  These will hopefully be used in the near future.

- Add back `with_const`, it would need to be added soon after anyway.
- Add back `split()` and `get_mut()`, they're useful.

compiler/rustc_middle/src/ty/mod.rs

@@ -1208,6 +1208,11 @@ pub trait WithConstness: Sized {
         ConstnessAnd { constness, value: self }
     }
 
+    #[inline]
+    fn with_const(self) -> ConstnessAnd<Self> {
+        self.with_constness(Constness::Const)
+    }
+
     #[inline]
     fn without_const(self) -> ConstnessAnd<Self> {
         self.with_constness(Constness::NotConst)