Fix plpgsql's handling of -- comments following expressions.

Up to now, read_sql_construct() has collected all the source text from
the statement or expression's initial token up to the character just
before the "until" token.  It normally tries to strip trailing
whitespace from that, largely for neatness.  If there was a "-- text"
comment after the expression, this resulted in removing the newline
that terminates the comment, which creates a hazard if we try to paste
the collected text into a larger SQL construct without inserting a
newline after it.  In particular this caused our handling of CASE
constructs to fail if there's a comment after a WHEN expression.

Commit 4adead1d2 noticed a similar problem with cursor arguments,
and worked around it through the rather crude hack of suppressing
the whitespace-trimming behavior for those.  Rather than do that
and leave the hazard open for future hackers to trip over, let's
fix it properly.  pl_scanner.c already has enough infrastructure
to report the end location of the expression's last token, so
we can copy up to that location and never collect any trailing
whitespace or comment to begin with.

Erik Wienhold and Tom Lane, per report from Michal Bartak.
Back-patch to all supported branches.

Discussion: https://postgr.es/m/CAAVzF_FjRoi8fOVuLCZhQJx6HATQ7MKm=aFOHWZODFnLmjX-xA@mail.gmail.com

src/test/regress/sql/plpgsql.sql

@@ -2047,11 +2047,9 @@ begin
 end $$ language plpgsql;
 select namedparmcursor_test7();
 
--- check that line comments work correctly within the argument list (there
--- is some special handling of this case in the code: the newline after the
--- comment must be preserved when the argument-evaluating query is
--- constructed, otherwise the comment effectively comments out the next
--- argument, too)
+-- check that line comments work correctly within the argument list
+-- (this used to require a special hack in the code; it no longer does,
+-- but let's keep the test anyway)
 create function namedparmcursor_test8() returns int4 as $$
 declare
   c1 cursor (p1 int, p2 int) for