[AST] Concurrency: Strip existential if constrained type was stripped down to a class

`any C` where `C` is a class is not a well-formed type.

(cherry picked from commit 10541ef)

Author: xedin

lib/AST/Type.cpp

@@ -989,6 +989,9 @@ Type TypeBase::stripConcurrency(bool recurse, bool dropGlobalActor) {
             existentialType->getConstraintType().getPointer())
       return Type(this);
 
+    if (newConstraintType->getClassOrBoundGenericClass())
+      return newConstraintType;
+
     return ExistentialType::get(newConstraintType);
   }