pgsql: Only allow typed tables to hang off composite types, not e.g. ta

Only allow typed tables to hang off composite types, not e.g. tables.

This also ensures that we take a relation lock on the composite type when
creating a typed table, which is necessary to prevent the composite type
and the typed table from getting out of step in the face of concurrent
DDL.

Noah Misch, with some changes.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/04db0fdbfa9382730bb65f94bca2cd8063a3456a

Modified Files
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src/backend/parser/parse_utilcmd.c        |   17 ++++++++++++++++-
src/test/regress/expected/typed_table.out |    2 ++
src/test/regress/sql/typed_table.sql      |    2 ++
3 files changed, 20 insertions(+), 1 deletions(-)