gc: use correct line number for EOF syntax error

I also added some debugging code that's turned
on with -xx.

Fixes #3392.

R=rsc, lvd
CC=golang-dev
https://golang.org/cl/5909058

src/cmd/gc/lex.c

@@ -1531,9 +1531,7 @@ getc(void)
 	if(c != 0) {
 		curio.peekc = curio.peekc1;
 		curio.peekc1 = 0;
-		if(c == '\n' && pushedio.bin == nil)
-			lexlineno++;
-		return c;
+		goto check;
 	}
 	
 	if(curio.bin == nil) {
@@ -1543,6 +1541,7 @@ getc(void)
 	} else
 		c = Bgetc(curio.bin);
 
+check:
 	switch(c) {
 	case 0:
 		if(curio.bin != nil) {

src/cmd/gc/subr.c

@@ -142,6 +142,11 @@ yyerror(char *fmt, ...)
 		if(debug['x'])	
 			print("yyerror: yystate=%d yychar=%d\n", yystate, yychar);
 
+		// An unexpected EOF caused a syntax error. Use the previous
+		// line number since getc generated a fake newline character.
+		if(curio.eofnl)
+			lexlineno = prevlineno;
+
 		// only one syntax error per line
 		if(lastsyntax == lexlineno)
 			return;

test/fixedbugs/bug435.go

+// errorcheck
+
+// Copyright 2012 The Go Authors.  All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Test that a syntax error caused by an unexpected EOF
+// gives an error message with the correct line number.
+//
+// https://code.google.com/p/go/issues/detail?id=3392
+
+package main
+
+func foo() {
+	bar(1, // ERROR "unexpected"