pgsql: Invent nodetag_only attribute for Nodes.

Invent nodetag_only attribute for Nodes.

This allows explaining gen_node_support.pl's handling of execnodes.h
and some other input files as being a shortcut for explicit marking
of all their node declarations as pg_node_attr(nodetag_only).
I foresee that someday we might need to be more fine-grained about
that, and this change provides the infrastructure needed to do so.
For now, it just allows removal of the script's klugy special case
for CallContext and InlineCodeBlock.

Discussion: https://postgr.es/m/75063.1657410615@sss.pgh.pa.us

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/ca187d7455f174da40e26e6e0c8361821ee19559

Modified Files
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src/backend/nodes/gen_node_support.pl | 27 ++++++++++++++++-----------
src/include/nodes/nodes.h | 5 +++++
src/include/nodes/parsenodes.h | 4 ++++
3 files changed, 25 insertions(+), 11 deletions(-)