[postgresql] Fix for #4306 -- ambiguity in target_el

[kaby76]

This is a fix for #4306.

Consider input select x || y from collate_test10;. This substring x || y, can be parsed two ways:

• target_el => a_expr identifier ==> x || y.
• target_el => a_expr ==> a_expr_qual_op => a_expr_unary_qualop qual_op a_expr_unary_qualop ==> x || y. This is the likely preferred derivation.

The fix is to add a semantic predicate before an alt in the rule a_expr_qual, which specifies the syntax for a unary postfix operator. Only ! and !! should be allowed.

However, PostgreSQL allows new operators to be declared. This will need to be fixed later on.

[kaby76]

PostgreSQL allows one to declare arbitrary operators with various args and precedence. I'm going to punt on this for now, but it should be fixed properly.

[teverett]

@@kaby76 thanks!