[PATCH, wide-int] change fixed_wide_int_storage from class to struct

[Gerald Pfeifer]

While not a bug according to the language of the C++ standard, this
causes dozens of warnings when building GCC with clang, and there is
not benefit of mixing struct and class like this.

Mike, when I had a similar case a while ago you sounded pretty 
supportive of this kind of change.  Would you be fine to pre-approve
such changes?

Gerald

2016-05-15  Gerald Pfeifer  <gerald@pfeifer.com>

	* wide-int.h: Change fixed_wide_int_storage from class to struct.

Index: wide-int.h
===================================================================
--- wide-int.h	(revision 236255)
+++ wide-int.h	(working copy)
@@ -294,7 +294,7 @@
   HOST_WIDE_INT *VAL = RESULT.write_val ()
 
 template <typename T> class generic_wide_int;
-template <int N> struct fixed_wide_int_storage;
+template <int N> class fixed_wide_int_storage;
 class wide_int_storage;
 
 /* An N-bit integer.  Until we can use typedef templates, use this instead.  */