pgsql: Mark PlanState as an abstract node type.

Mark PlanState as an abstract node type.

In the same vein as commit 251154beb, make it clear that we never
instantiate PlanState.

Also mark MemoryContextData as abstract.  This has no effect right now,
since memnodes.h isn't one of the files fed to gen_node_support.pl.
But it seems like good documentation and future-proofing.

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/b4f79d278f2a78a65ff020b4e8cfa46a2587091d

Modified Files
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src/include/nodes/execnodes.h                    | 2 ++
src/include/nodes/memnodes.h                     | 2 ++
src/test/modules/test_oat_hooks/test_oat_hooks.c | 3 ---
3 files changed, 4 insertions(+), 3 deletions(-)