Implement better handling of divergence

Divergence here means that for some reason, the end of a block will not be
reached. We tried to model this just using the never type, but that doesn't work
fully (e.g. in `let x = { loop {}; "foo" };` x should still have type `&str`);
so this introduces a `diverges` flag that the type checker keeps track of, like
rustc does.

crates/ra_hir_ty/src/lib.rs

@@ -730,6 +730,13 @@ impl Ty {
         }
     }
 
+    pub fn is_never(&self) -> bool {
+        match self {
+            Ty::Apply(ApplicationTy { ctor: TypeCtor::Never, .. }) => true,
+            _ => false,
+        }
+    }
+
     /// If this is a `dyn Trait` type, this returns the `Trait` part.
     pub fn dyn_trait_ref(&self) -> Option<&TraitRef> {
         match self {