pgsql: Before attempting to create a composite type, check whether a

Log Message:
-----------
Before attempting to create a composite type, check whether a type of that
name already exists, so we'd get an error message about a "type" instead
of about a "relation", because the composite type code shares code with
relation creation.

Modified Files:
--------------
    pgsql/src/backend/commands:
        typecmds.c (r1.145 -> r1.146)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/typecmds.c?r1=1.145&r2=1.146)