gdb: add type::is_flag_enum / type::set_is_flag_enum

gdb: add type::is_flag_enum / type::set_is_flag_enum

Add the `is_flag_enum` and `set_is_flag_enum` methods on `struct type`,
in order to remove the `TYPE_FLAG_ENUM` macro.  In this patch, the macro
is changed to use the getter, so all the call sites of the macro that
are used as a setter are changed to use the setter method directly.  The
next patch will remove the macro completely.

gdb/ChangeLog:

	* gdbtypes.h (struct type) <is_flag_enum,
	set_is_flag_enum>: New methods.
	(TYPE_FLAG_ENUM): Use type::is_flag_enum, change all
	write call sites to use type::set_is_flag_enum.

Change-Id: I9c56c91626c8d784947ba94fcb97818526b81d1c