C++: Prevent incomplete classes from being Types

Raw classes from the database that are incomplete and should be
represented by their complete twin are now allowed to be `Element`s for
performance reasons, but this commit prevents them from being `Type`s.
It was causing confusion in test results and might also cause confusion
in queries.

cpp/ql/src/semmle/code/cpp/Class.qll

@@ -12,8 +12,7 @@ private import semmle.code.cpp.internal.ResolveClass
  */
 class Class extends UserType {
   Class() {
-    isClass(this.underlying()) and
-    this = resolveClass(_)
+    isClass(this.underlying())
   }
 
   override @element unresolve() {

cpp/ql/src/semmle/code/cpp/Type.qll

@@ -7,6 +7,8 @@ private import semmle.code.cpp.internal.ResolveClass
  * A C/C++ type.
  */
 class Type extends Locatable, @type {
+  Type() { isType(this.underlying()) }
+
   /**
    * Gets the name of this type.
    */

cpp/ql/src/semmle/code/cpp/internal/ResolveClass.qll

@@ -54,4 +54,10 @@ cached private module Cached {
     (usertypes(t,_,1) or usertypes(t,_,2) or usertypes(t,_,3) or usertypes(t,_,6)
     or usertypes(t,_,10) or usertypes(t,_,11) or usertypes(t,_,12))
   }
+
+  cached predicate isType(@type t) {
+    not isClass(t)
+    or
+    t = resolveClass(_)
+  }
 }