Use int instead of LONGEST in tdesc_type sizes.

Use int instead of LONGEST in tdesc_type sizes.

gdb/ChangeLog:

	* target-descriptions.c (struct tdesc_type) <u.u.size>: Change type
	from LONGEST to int.
	(struct tdesc_type) <u.f.size>: Ditto.
	(tdesc_set_struct_size): Change type of "size" arg from LONGEST
	to int.  Add assertion size > 0.
	(tdesc_create_flags): Ditto.
	* target-descriptions.h (tdesc_set_struct_size): Update.
	(tdesc_create_flags): Update.
	* xml-tdesc.c (MAX_FIELD_SIZE, MAX_FIELD_BITSIZE): New macros.
	(MAX_VECTOR_SIZE): New macro.
	(tdesc_start_struct): Catch conversion errors from LONGEST to int.
	(tdesc_start_flags, tdesc_start_field, tdesc_start_vector): Ditto.