C++: Make Declaration not abstract

It looks like this change will stop `SignedOverflowCheck.ql` from
needlessly re-evaluating several cached stages.

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

@@ -25,7 +25,7 @@ private import semmle.code.cpp.internal.QualifiedName as Q
  * `DeclarationEntry`, because they always have a unique source location.
  * `EnumConstant` and `FriendDecl` are both examples of this.
  */
-abstract class Declaration extends Locatable, @declaration {
+class Declaration extends Locatable, @declaration {
   /**
    * Gets the innermost namespace which contains this declaration.
    *