pgsql: Don't allow CREATE TABLE AS to create a column with invalid coll

Don't allow CREATE TABLE AS to create a column with invalid collation

It is possible that an expression ends up with a collatable type but
without a collation.  CREATE TABLE AS could then create a table based
on that.  But such a column cannot be dumped with valid SQL syntax, so
we disallow creating such a column.

per test report from Noah Misch

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/b9cff97fdf486eca7e563a9696a7391048814d0f

Modified Files
--------------
src/backend/catalog/heap.c                       |   17 +++++++++++++++--
src/backend/catalog/index.c                      |    8 +++++---
src/backend/commands/tablecmds.c                 |    4 ++--
src/include/catalog/heap.h                       |    2 +-
src/test/regress/expected/collate.linux.utf8.out |    3 +++
src/test/regress/sql/collate.linux.utf8.sql      |    2 ++
6 files changed, 28 insertions(+), 8 deletions(-)