Turn remaining value_contents functions into methods

This turns the remaining value_contents functions -- value_contents,
value_contents_all, value_contents_for_printing, and
value_contents_for_printing_const -- into methods of value.  It also
converts the static functions require_not_optimized_out and
require_available to be private methods.

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

gdb/ada-tasks.c

@@ -430,7 +430,7 @@ iterate_over_live_ada_tasks (ada_task_list_iterator_ftype iterator)
 static void
 value_as_string (char *dest, struct value *val, int length)
 {
-  memcpy (dest, value_contents (val).data (), length);
+  memcpy (dest, val->contents ().data (), length);
   dest[length] = '\0';
 }