CPlusPlus: Allow " = default" also on function implementations

Note that we only make sure not to trip over valid code; we make no
effort to check whether default/delete is actually allowed at this
type of declaration.

Fixes: QTCREATORBUG-28102
Change-Id: Ic693319b9dfaf8652cf4cae9cd907a6e258ad773
Reviewed-by: <github-actions-qt-creator@cristianadam.eu>
Reviewed-by: Christian Stenger <christian.stenger@qt.io>

tests/auto/cplusplus/cxx11/data/defaultdeleteInitializer.1.cpp

@@ -1,8 +1,10 @@
 class C {
-  C() = default;
+  C();
   C(const C &) = delete;
   C &operator=(const C &) = default;
 
   void foo() = delete;
   template <class T> void bar(T) = delete;
 };
+
+C::C() = default;