pgsql: Don't create a shell type for function returning an array

Don't create a shell type for function returning an array

Refactor the checks in the function to move all the conditions for
when to attempt creating a shell type into one place. Add a check for
the array syntax.

In addition to rejecting array syntax, another user-visible effect is
that the error message is now different if the type specified a
typmod. You now get "type does not exist" instead of the more
specific "type modifier cannot be specified for shell type". That
seems better; the implicit shell type creation exists only for
backwards compatibility, and it never worked with type modifiers, so
if there's a type modifier it's most likely not because the user tried
to create a shell type,

Add test for the array syntax, the type modifier, and some other cases
for which we don't create shell types.

Discussion: https://www.postgresql.org/message-id/de673feb-41b4-4685-b24b-6408b95e58ab@iki.fi
Backpatch-through: 14

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/2866d8c7dbfc9d882a7d80fef93fbbe763709932

Modified Files
--------------
src/backend/commands/functioncmds.c | 58 +++++++++++++++++++------------
src/test/regress/expected/create_type.out | 50 +++++++++++++++++++++++++-
src/test/regress/sql/create_type.sql | 42 ++++++++++++++++++++++
3 files changed, 126 insertions(+), 24 deletions(-)