gdb: remove TYPE_FIELD macro

gdb: remove TYPE_FIELD macro

Replace all uses of it by type::field.

Note that since type::field returns a reference to the field, some spots
are used to assign the whole field structure.  See ctfread.c, function
attach_fields_to_type, for example.  This is the same as was happening
with the macro, so I don't think it's a problem, but if anybody sees a
really nicer way to do this, now could be a good time to implement it.

gdb/ChangeLog:

	* gdbtypes.h (TYPE_FIELD): Remove.  Replace all uses with
	type::field.