Read a structure from its definition, not from a declaration before it

[estebanzimanyi]

A structure the headers declare before they define it is read from its
definition: its record keeps the place where the parse first meets the
structure, and its fields, offsets and file are those of the definition.
A structure the headers only declare, an opaque one such as NumericData,
keeps its declaration and states no field.

Why. A binding laying a structure out reads its fields and offsets from
the catalog. meos.h declares MeosArray and SkipList, which
meos_internal.h defines; read from the first record the parse meets, the
declaration, they carry no field, and neither would varlena once
pg_basetypes.h declares struct varlena ahead of the splice defining it,
which MobilityDB #2893 does.

Measured. Over MobilityDB d5e3e9946e, three structures gain their
fields and nothing else changes: MeosArray five, read in meos_internal.h
for meos.h, SkipList fourteen, read in meos_internal.h for meos.h, and
PCSCHEMA eleven, read in pc_api.h for meos_pointcloud.h. Over MobilityDB
#2893 (dd0426728b) varlena keeps its two fields, and that catalog differs
from master's only in where its structures and typedefs sit: NumericData
is found in pg_basetypes.h instead of pg_numeric.h, so it no longer reads
as vendored, and the structures come in another order.

Witness. tests/test_struct_layout.py reads MeosArray, SkipList and
varlena with their fields from the catalog, and parses a header
declaring one structure before a second header defines it, beside one
only declared: the first carries its fields and the second none. The
suite floor goes from 341 to 343.