C++ front-end: Type check Apple block pointers

Fixes: #1647

Author: tautschnig

regression/cpp/Apple_extensions1/test.desc

@@ -1,10 +1,8 @@
-KNOWNBUG
+CORE
 main.cpp
 
 ^EXIT=0$
 ^SIGNAL=0$
 --
 ^warning: ignoring
 ^CONVERSION ERROR$
---
-This is being tracked in #1647.

src/cpp/cpp_typecheck_type.cpp

@@ -252,6 +252,9 @@ void cpp_typecheckt::typecheck_type(typet &type)
   {
     // This is an Apple extension for lambda-like constructs.
     // http://thirdcog.eu/pwcblocks/
+    // we just treat them as references to functions
+    type.id(ID_frontend_pointer);
+    typecheck_type(type);
   }
   else if(type.id()==ID_nullptr)
   {