Allow ALTER TYPE to update an existing type's typsubscript value.

This is essential if we'd like to allow existing extension data types
to support subscripting in future, since dropping and recreating the
type isn't a practical thing for an extension upgrade script, and
direct manipulation of pg_type isn't a great answer either.

There was some discussion about also allowing alteration of typelem,
but it's less clear whether that's a good idea or not, so for now
I forebore.

Discussion: https://postgr.es/m/3724341.1607551174@sss.pgh.pa.us

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