Attempt to get plpython regression tests working again for MSVC builds.

This has probably been broken for quite a long time. Buildfarm member
currawong's current results suggest that it's been broken since 9.1, so
backpatch this to that branch.

This only supports Python 2 - I will handle Python 3 separately, but
this is a fairly simple fix.

src/tools/msvc/vcregress.pl

@@ -170,8 +170,15 @@ sub plcheck
 	{
 		next unless -d "$pl/sql" && -d "$pl/expected";
 		my $lang = $pl eq 'tcl' ? 'pltcl' : $pl;
+		if ($lang eq 'plpython')
+		{
+			next unless -d "../../$Config/plpython2";
+			$lang = 'plpythonu';
+		}
+		else
+		{
 			next unless -d "../../$Config/$lang";
-		$lang = 'plpythonu' if $lang eq 'plpython';
+		}
 		my @lang_args = ("--load-extension=$lang");
 		chdir $pl;
 		my @tests = fetchTests();