C++: Ensure we can round trip between (forward) class declarations

This was already possible when the forward class declaration and the class
definition occurred in the same scope. However, there is a common C++ usage
pattern in which this is not the case (when only a pointer to the class is
needed). In this latter scenario we could not round trip between the (forward)
`DeclarationEntry` and the `Declaration`.

Effectively this changes the code to:
```
if exists(TypeDeclarationEntry e | e.getType() = this)
then result.getType() = this
else ...
```
We use `type_decls` instead to stay close to the original code.

Author: jketema

cpp/ql/lib/semmle/code/cpp/UserType.qll

@@ -48,8 +48,8 @@ class UserType extends Type, Declaration, NameQualifyingElement, AccessHolder, @
   }
 
   override TypeDeclarationEntry getADeclarationEntry() {
-    if type_decls(_, underlyingElement(this), _)
-    then type_decls(unresolveElement(result), underlyingElement(this), _)
+    if type_decls(_, unresolveElement(this), _)
+    then type_decls(underlyingElement(result), unresolveElement(this), _)
     else exists(Class t | this.(Class).isConstructedFrom(t) and result = t.getADeclarationEntry())
   }
 

cpp/ql/test/library-tests/declarationEntry/declarationEntry/declarationEntry.expected

@@ -30,7 +30,7 @@
 | declarationEntry.cpp:39:7:39:7 | operator= | declarationEntry.cpp:39:7:39:7 | declaration of operator= | 1 | 1 |
 | declarationEntry.cpp:39:7:39:7 | operator= | declarationEntry.cpp:39:7:39:7 | declaration of operator= | 1 | 1 |
 | declarationEntry.cpp:39:7:39:13 | myClass | declarationEntry.cpp:39:7:39:13 | definition of myClass | 1 | 1 |
-| declarationEntry.cpp:39:7:39:13 | myClass | forwardDeclaration.cpp:1:7:1:13 | declaration of myClass | 0 | 1 |
+| declarationEntry.cpp:39:7:39:13 | myClass | forwardDeclaration.cpp:1:7:1:13 | declaration of myClass | 1 | 1 |
 | declarationEntry.cpp:42:6:42:21 | myMemberVariable | declarationEntry.cpp:42:6:42:21 | definition of myMemberVariable | 1 | 1 |
 | forwardDeclaration.cpp:3:10:3:19 | myClassPtr | forwardDeclaration.cpp:3:10:3:19 | definition of myClassPtr | 1 | 1 |
 | macro.c:2:1:2:3 | foo | macro.c:2:1:2:3 | declaration of foo | 1 | 1 |

cpp/ql/test/library-tests/declarationEntry/declarationEntry/roundTrip.expected

@@ -39,7 +39,7 @@
 | file://:0:0:0:0 | definition of gp_offset | file://:0:0:0:0 | gp_offset | yes |
 | file://:0:0:0:0 | definition of overflow_arg_area | file://:0:0:0:0 | overflow_arg_area | yes |
 | file://:0:0:0:0 | definition of reg_save_area | file://:0:0:0:0 | reg_save_area | yes |
-| forwardDeclaration.cpp:1:7:1:13 | declaration of myClass | declarationEntry.cpp:39:7:39:13 | myClass | no |
+| forwardDeclaration.cpp:1:7:1:13 | declaration of myClass | declarationEntry.cpp:39:7:39:13 | myClass | yes |
 | forwardDeclaration.cpp:3:10:3:19 | definition of myClassPtr | forwardDeclaration.cpp:3:10:3:19 | myClassPtr | yes |
 | macro.c:2:1:2:3 | declaration of foo | macro.c:2:1:2:3 | foo | yes |
 | macro.c:4:5:4:8 | definition of main | macro.c:4:5:4:8 | main | yes |