gnu-v2-abi.c: Add casts

gnu-v2-abi.c: Add casts

I looked at changing these is_destructor_name/is_constructor_name
interfaces in order to detangle the boolean result from the ctor/dtor
kind return, but then realized that this design goes all the way down
to the libiberty demangler interfaces.  E.g, include/demangle.h:

 ~~~
 /* Return non-zero iff NAME is the mangled form of a constructor name
    in the G++ V3 ABI demangling style.  Specifically, return an `enum
    gnu_v3_ctor_kinds' value indicating what kind of constructor
    it is.  */
 extern enum gnu_v3_ctor_kinds
	 is_gnu_v3_mangled_ctor (const char *name);

 enum gnu_v3_dtor_kinds {
   gnu_v3_deleting_dtor = 1,
   gnu_v3_complete_object_dtor,
   gnu_v3_base_object_dtor,
   /* These are not part of the V3 ABI.  Unified destructors are generated
      as a speed-for-space optimization when the -fdeclone-ctor-dtor option
      is used, and are always internal symbols.  */
   gnu_v3_unified_dtor,
   gnu_v3_object_dtor_group
 };
 ~~~

libiberty/cp-demangle.c:

 ~~~
 enum gnu_v3_ctor_kinds
 is_gnu_v3_mangled_ctor (const char *name)
 {
   enum gnu_v3_ctor_kinds ctor_kind;
   enum gnu_v3_dtor_kinds dtor_kind;

   if (! is_ctor_or_dtor (name, &ctor_kind, &dtor_kind))
     return (enum gnu_v3_ctor_kinds) 0;
   return ctor_kind;
 }
 ~~~

etc.

gdb/ChangeLog:
2015-10-29  Pedro Alves  <palves@redhat.com>

	* gnu-v2-abi.c (gnuv2_is_destructor_name)
	(gnuv2_is_constructor_name): Add casts.