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
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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(-)