pgsql: Allow for arbitrary data types as content in XMLELEMENT.

Log Message:
-----------
Allow for arbitrary data types as content in XMLELEMENT.  The original
coercion to type xml was a mistake.  Escape values so they are valid
XML character data.

Modified Files:
--------------
    pgsql/src/backend/parser:
        parse_expr.c (r1.205 -> r1.206)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/parser/parse_expr.c.diff?r1=1.205&r2=1.206)
    pgsql/src/backend/utils/adt:
        xml.c (r1.14 -> r1.15)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/xml.c.diff?r1=1.14&r2=1.15)
    pgsql/src/test/regress/expected:
        xml.out (r1.5 -> r1.6)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/expected/xml.out.diff?r1=1.5&r2=1.6)
        xml_1.out (r1.6 -> r1.7)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/expected/xml_1.out.diff?r1=1.6&r2=1.7)
    pgsql/src/test/regress/sql:
        xml.sql (r1.5 -> r1.6)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/sql/xml.sql.diff?r1=1.5&r2=1.6)