gc: fix weird error message

Fixes #1670.

R=ken2
CC=golang-dev
https://golang.org/cl/4386045

src/cmd/gc/subr.c

@@ -135,6 +135,7 @@ yyerror(char *fmt, ...)
 	int i;
 	static int lastsyntax;
 	va_list arg;
+	char buf[512], *p;
 
 	if(strncmp(fmt, "syntax error", 12) == 0) {
 		nsyntaxerrors++;
@@ -147,6 +148,16 @@ yyerror(char *fmt, ...)
 			return;
 		lastsyntax = lexlineno;
 		
+		if(strstr(fmt, "{ or {")) {
+			// The grammar has { and LBRACE but both show up as {.
+			// Rewrite syntax error referring to "{ or {" to say just "{".
+			strecpy(buf, buf+sizeof buf, fmt);
+			p = strstr(buf, "{ or {");
+			if(p)
+				memmove(p+1, p+6, strlen(p+6)+1);
+			fmt = buf;
+		}
+		
 		// look for parse state-specific errors in list (see go.errors).
 		for(i=0; i<nelem(yymsg); i++) {
 			if(yymsg[i].yystate == yystate && yymsg[i].yychar == yychar) {