C++: Make unresolve dispatch on result, not this

This change means that there are no results for `unresolveElement(t)`
where `t` is a "junk type" -- a class definition that is not in the
image of `resolveClass`. These "junk types" still exist as `Element`s,
but they will never be returned by any predicate that goes through
`unresolveElement` to query the db.

We get a small reduction in DIL size and a significant speed
improvement. The DIL for `NewArrayDeleteMismatch.ql` is reduced from
27,630 lines to 27,507 lines, and the total analysis time for the LGTM
suite on jdk8u is reduced from 1158s to 984s.

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

@@ -15,10 +15,6 @@ class Class extends UserType {
     isClass(this.underlying())
   }
 
-  override @element unresolve() {
-    resolveClass(result) = this
-  }
-
   /** Gets a child declaration of this class. */
   override Declaration getADeclaration() { result = this.getAMember() }
 

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

@@ -70,7 +70,12 @@ class ElementBase extends @element {
    * See `underlying` for when the qualifier is `this`.
    */
   pragma[inline]
-  @element unresolve() { result = this }
+  final @element unresolve() {
+    not result instanceof @usertype and
+    result = this
+    or
+    this = resolveClass(result)
+  }
 }
 
 /**