[Completion] Assert we have a type in getClosureActorIsolation

Previously we could end up in cases where we pick the wrong
DeclContext in `TypeCheckASTNodeAtLocRequest` since we previously
weren't checking source ranges, which could result in skipping the
type-checking of an outer closure. Now that we correctly pick the
DeclContext, we should no longer hit that case, so we should be able
to fall into `getTypeForCompletion` and assert that the solution has
a type. Also while here let's upgrade that assert to a
`CONDITIONAL_ASSERT`.

Author: hamishknight

lib/IDE/PostfixCompletion.cpp

@@ -110,9 +110,6 @@ getClosureActorIsolation(const Solution &S, AbstractClosureExpr *ACE) {
       if (auto Ty = target->getClosureContextualType())
         return Ty;
     }
-    if (!S.hasType(E)) {
-      return Type();
-    }
     return getTypeForCompletion(S, E);
   };
   auto getClosureActorIsolationThunk = [&S](AbstractClosureExpr *ACE) {

lib/IDE/TypeCheckCompletionCallback.cpp

@@ -58,7 +58,7 @@ Type swift::ide::getTypeForCompletion(const constraints::Solution &S,
   }
 
   if (!S.hasType(Node)) {
-    assert(false && "Expression wasn't type checked?");
+    CONDITIONAL_ASSERT(false && "Expression wasn't type checked?");
     return nullptr;
   }