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_mir/src/interpret/machine.rs

@@ -71,6 +71,11 @@ pub trait AllocMap<K: Hash + Eq, V> {
     fn get(&self, k: K) -> Option<&V> {
         self.get_or(k, || Err(())).ok()
     }
+
+    /// Mutable lookup.
+    fn get_mut(&mut self, k: K) -> Option<&mut V> {
+        self.get_mut_or(k, || Err(())).ok()
+    }
 }
 
 /// Methods of this trait signifies a point where CTFE evaluation would fail