pgsql: Revert ill-considered change to plpgsql: it should not rely on

Log Message:
-----------
Revert ill-considered change to plpgsql: it should not rely on the
current setting of standard_conforming_strings to decide how to quote
strings that will be used later.  There is much more to do here but
this particular change breaks the build on Windows, so fix it now.

Modified Files:
--------------
    pgsql/src/pl/plpgsql/src:
        gram.y (r1.89 -> r1.90)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/pl/plpgsql/src/gram.y.diff?r1=1.89&r2=1.90)