cmd/yacc: fix handling of tokens that don't start with letters

CL 149110043 changed yacc to no longer keep a leading space
for quoted tokens.  That is OK by itself but unfortunately
yacc was relying on that leading space to notice which tokens
it should not output as const declarations.

Add a few such tokens to expr.y, although it won't make any
immediate difference as we seem to have no tests for yacc.

LGTM=rsc
R=golang-codereviews, rsc
CC=golang-codereviews
https://golang.org/cl/152720043

src/cmd/yacc/testdata/expr/expr.y

@@ -32,6 +32,8 @@ import (
 
 %type	<num>	expr expr1 expr2 expr3
 
+%token '+' '-' '*' '/' '(' ')'
+
 %token	<num>	NUM
 
 %%

src/cmd/yacc/yacc.go

@@ -195,8 +195,9 @@ type Item struct {
 }
 
 type Symb struct {
-	name  string
-	value int
+	name    string
+	noconst bool
+	value   int
 }
 
 type Wset struct {
@@ -509,8 +510,7 @@ outer:
 	// put out non-literal terminals
 	for i := TOKSTART; i <= ntokens; i++ {
 		// non-literals
-		c := tokset[i].name[0]
-		if c != ' ' && c != '$' {
+		if !tokset[i].noconst {
 			fmt.Fprintf(ftable, "const %v = %v\n", tokset[i].name, tokset[i].value)
 		}
 	}
@@ -734,7 +734,7 @@ func defin(nt int, s string) int {
 			copy(anontrst, nontrst)
 			nontrst = anontrst
 		}
-		nontrst[nnonter] = Symb{s, 0}
+		nontrst[nnonter] = Symb{name: s}
 		return NTBASE + nnonter
 	}
 
@@ -769,9 +769,13 @@ func defin(nt int, s string) int {
 		if val == 0 {
 			errorf("token value 0 is illegal")
 		}
+		tokset[ntokens].noconst = true
 	} else {
 		val = extval
 		extval++
+		if s[0] == '$' {
+			tokset[ntokens].noconst = true
+		}
 	}
 
 	tokset[ntokens].value = val