Turn some value_contents functions into methods

This turns value_contents_raw, value_contents_writeable, and
value_contents_all_raw into methods on value.  The remaining functions
will be changed later in the series; they were a bit trickier and so I
didn't include them in this patch.

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

gdb/dwarf2/loc.c

@@ -1363,8 +1363,8 @@ value_of_dwarf_reg_entry (struct type *type, frame_info_ptr frame,
 				 release_value (target_val).release ());
 
   /* Copy the referencing pointer to the new computed value.  */
-  memcpy (value_contents_raw (val).data (),
-	  value_contents_raw (outer_val).data (),
+  memcpy (val->contents_raw ().data (),
+	  outer_val->contents_raw ().data (),
 	  checked_type->length ());
   val->set_lazy (0);