Base information_schema.sql_identifier domain on name, not varchar.

The SQL spec says that sql_identifier is a domain over varchar,
but it also says that that domain is supposed to represent the set
of valid identifiers for the implementation, in particular applying
a length limit matching the implementation's identifier length limit.
We were declaring sql_identifier as just "character varying", thus
duplicating what the spec says about base type, but entirely failing
at the rest of it.

Instead, let's declare sql_identifier as a domain over type "name".
(We can drop the COLLATE "C" added by commit 6b0faf723, since that's
now implicit in "name".)  With the recent improvements to name's
comparison support, there's not a lot of functional difference between
name and varchar.  So although in principle this is a spec deviation,
it's a pretty minor one.  And correctly enforcing PG's name length limit
is a good thing; on balance this seems closer to the intent of the spec
than what we had.

But that's all just language-lawyering.  The *real* reason to do this is
that it makes sql_identifier columns exposed by information_schema views
be just direct representations of the underlying "name" catalog columns,
eliminating a semantic mismatch that was disastrous for performance of
typical queries on the information_schema.  In combination with the
recent change to allow dropping no-op CoerceToDomain nodes, this allows
(for example) queries such as

    select ... from information_schema.tables where table_name = 'foo';

to produce an indexscan rather than a seqscan on pg_class.

Discussion: https://postgr.es/m/CAFj8pRBUCX4LZ2rA2BbEkdD6NN59mgx+BLo1gO08Wod4RLtcTg@mail.gmail.com

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