Turn value_fetch_lazy into a method

This changes value_fetch_lazy to be a method of value.  A few helper
functions are converted as well, to avoid problems in later patches
when the data members are all made private.

Approved-By: Simon Marchi <simon.marchi@efficios.com>

gdb/gnu-v2-abi.c

@@ -164,7 +164,7 @@ gnuv2_virtual_fn_field (struct value **arg1p, struct fn_field * f, int j,
       if (!arg1->lazy ())
 	{
 	  arg1->set_lazy (1);
-	  value_fetch_lazy (arg1);
+	  arg1->fetch_lazy ();
 	}
 
       vfn = value_field (entry, 2);