cmd/gc: better error messages for C-style if statements.

Given
        if (i == 0)
                x++
The old message was
        x.go:6: syntax error: unexpected semicolon or newline before {
Now we see
        x.go:6: syntax error: missing { after if clause

Fixes #5687

R=golang-dev, adg
CC=golang-dev
https://golang.org/cl/12822045

src/cmd/gc/go.errors

@@ -21,16 +21,16 @@ static struct {
 	"missing import path; require quoted string",
 
 	% loadsys package imports LFUNC LNAME '(' ')' '{' LIF if_header ';'
-	"unexpected semicolon or newline before {",
+	"missing { after if clause",
 
 	% loadsys package imports LFUNC LNAME '(' ')' '{' LSWITCH if_header ';'
-	"unexpected semicolon or newline before {",
+	"missing { after switch clause",
 
 	% loadsys package imports LFUNC LNAME '(' ')' '{' LFOR for_header ';'
-	"unexpected semicolon or newline before {",
+	"missing { after for clause",
 
 	% loadsys package imports LFUNC LNAME '(' ')' '{' LFOR ';' LBODY
-	"unexpected semicolon or newline before {",
+	"missing { after for clause",
 
 	% loadsys package imports LFUNC LNAME '(' ')' ';' '{'
 	"unexpected semicolon or newline before {",

src/cmd/gc/y.tab.h

-/* A Bison parser, made by GNU Bison 2.5.  */
+/* A Bison parser, made by GNU Bison 2.3.  */
 
-/* Bison interface for Yacc-like parsers in C
+/* Skeleton interface for Bison's Yacc-like parsers in C
 
-      Copyright (C) 1984, 1989-1990, 2000-2011 Free Software Foundation, Inc.
+   Copyright (C) 1984, 1989, 1990, 2000, 2001, 2002, 2003, 2004, 2005, 2006
+   Free Software Foundation, Inc.
 
-   This program is free software: you can redistribute it and/or modify
+   This program is free software; you can redistribute it and/or modify
    it under the terms of the GNU General Public License as published by
-   the Free Software Foundation, either version 3 of the License, or
-   (at your option) any later version.
+   the Free Software Foundation; either version 2, or (at your option)
+   any later version.
 
    This program is distributed in the hope that it will be useful,
    but WITHOUT ANY WARRANTY; without even the implied warranty of
@@ -15,7 +16,9 @@
    GNU General Public License for more details.
 
    You should have received a copy of the GNU General Public License
-   along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
+   along with this program; if not, write to the Free Software
+   Foundation, Inc., 51 Franklin Street, Fifth Floor,
+   Boston, MA 02110-1301, USA.  */
 
 /* As a special exception, you may create a larger work that contains
    part or all of the Bison parser skeleton and distribute that work
@@ -30,7 +33,6 @@
    This special exception was added by the Free Software Foundation in
    version 2.2 of Bison.  */
 
-
 /* Tokens.  */
 #ifndef YYTOKENTYPE
 # define YYTOKENTYPE
@@ -144,28 +146,22 @@
 
 #if ! defined YYSTYPE && ! defined YYSTYPE_IS_DECLARED
 typedef union YYSTYPE
-{
-
-/* Line 2068 of yacc.c  */
 #line 30 "go.y"
-
+{
 	Node*		node;
 	NodeList*		list;
 	Type*		type;
 	Sym*		sym;
 	struct	Val	val;
 	int		i;
-
-
-
-/* Line 2068 of yacc.c  */
-#line 163 "y.tab.h"
-} YYSTYPE;
-# define YYSTYPE_IS_TRIVIAL 1
+}
+/* Line 1529 of yacc.c.  */
+#line 160 "y.tab.h"
+	YYSTYPE;
 # define yystype YYSTYPE /* obsolescent; will be withdrawn */
 # define YYSTYPE_IS_DECLARED 1
+# define YYSTYPE_IS_TRIVIAL 1
 #endif
 
 extern YYSTYPE yylval;
 
-

src/cmd/gc/yerr.h

@@ -21,16 +21,16 @@ static struct {
 	"missing import path; require quoted string",
 
 	377, ';',
-	"unexpected semicolon or newline before {",
+	"missing { after if clause",
 
 	398, ';',
-	"unexpected semicolon or newline before {",
+	"missing { after switch clause",
 
 	237, ';',
-	"unexpected semicolon or newline before {",
+	"missing { after for clause",
 
 	475, LBODY,
-	"unexpected semicolon or newline before {",
+	"missing { after for clause",
 
 	22, '{',
 	"unexpected semicolon or newline before {",