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.