gdb: add type::is_fixed_instance / type::set_is_fixed_instance

Add the `is_fixed_instance` and `set_is_fixed_instance` methods on `struct
type`, in order to remove the `TYPE_FIXED_INSTANCE` 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_fixed_instance,
	set_is_fixed_instance>: New methods.
	(TYPE_FIXED_INSTANCE): Use type::is_fixed_instance, change all
	write call sites to use type::set_is_fixed_instance.

Change-Id: I4401d81512fab9eab4232bbea48ce6c7d586b94c