enum_flags: Use C++11 std::underlying_type

Now that we can require C++11, we can use std::underlying_type instead
of rolling our own.

gdb/ChangeLog:
yyyy-mm-dd  Pedro Alves  <palves@redhat.com>

	* common/enum-flags.h: Include <type_traits>.
	(integer_for_size, enum_underlying_type): Delete.
	(class enum_flags): Use std::underlying_type.