pgsql: Fix the implicit-RTE code to be able to handle implicit RTEs for

Log Message:
-----------
Fix the implicit-RTE code to be able to handle implicit RTEs for CTEs, as
well as regular tables. Per discussion, this seems necessary to meet the
principle of least astonishment.

In passing, simplify the error messages in warnAutoRange(). Now that we
have parser error position info for these errors, it doesn't seem very
useful to word the error message differently depending on whether we are
inside a sub-select or not.

Modified Files:
--------------
pgsql/src/backend/parser:
parse_clause.c (r1.180 -> r1.181)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/parser/parse_clause.c?r1=1.180&r2=1.181)
parse_relation.c (r1.136 -> r1.137)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/parser/parse_relation.c?r1=1.136&r2=1.137)
pgsql/src/include/parser:
parse_relation.h (r1.59 -> r1.60)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/parser/parse_relation.h?r1=1.59&r2=1.60)