sql/pgwire: "char" datatype is not used correctly if selecting both a constant and from a table

[rafiss]

This issue breaks class generation functionality in the .NET EntityFramework Core ORM.

Consider this query: select 'c'::"char";

Using Wireshark, I found that in CockroachDB, the result comes back with a type OID of 18 -- the OID for the "char" datatype. This is correct.

However, consider the following:

root@:26257/defaultdb> create table  tab (a int primary key, b "char");
CREATE TABLE

root@:26257/defaultdb> select a,b, ''::"char" from tab;
  a | b | char
----+---+-------
(0 rows)

Now the ''::"char" value comes back with a type OID of 25 -- the OID for the text datatype. This is incorrect. It should still have a type OID of 18. The b column has the correct type OID of 18.

Also, in CockroachDB, the wrong typlen is returned for the "char" type. It should be 1, but it is -1 right now. Moved this to a separate issue: #48565

As a possible aside: it looks like the pg_catalog tables all use CHAR types when instead they should be using "char".