adjust concat precedence to match that of string comparison

adjust concat precedence to match that of string comparison

Adjusted the operator precedence for the string concatenation operator to
be equal to that of string matching operators, such as
:meth:`.ColumnElement.like`, :meth:`.ColumnElement.regexp_match`,
:meth:`.ColumnElement.match`, etc., as well as plain ``==`` which has the
same precedence as string comparison operators, so that parenthesis will be
applied to a string concatenation expression that follows a string match
operator. This provides for backends such as PostgreSQL where the "regexp
match" operator is apparently of higher precedence than the string
concatenation operator.

Fixes: #9610
Change-Id: I73640e40e445375177340e1ed8f45b5da98d6dfb