cmd/yacc: adjust expansion of $n to be more useful in errors

When the compiler echoes back an expression, it shows the
generated yacc expression. Change the generated code to
use a slice so that $3 shows up as yyDollar[3] in such messages.

Consider changing testdata/expr/expr.y to say:

	$$.Sub(float64($1), $3)

(The float64 conversion is incorrect.)

Before:
expr.y:70[expr.go:486]: cannot convert exprS[exprpt - 2].num (type *big.Rat) to type float64

After:
expr.y:70[expr.go:492]: cannot convert exprDollar[1].num (type *big.Rat) to type float64

Change-Id: I74e494069df588e62299d1fccb282f3658d8f8f4
Reviewed-on: https://go-review.googlesource.com/4630Reviewed-by: Rob Pike <r@golang.org>

src/cmd/yacc/yacc.go

@@ -603,6 +603,7 @@ outer:
 			}
 			levprd[nprod] |= ACTFLAG
 			fmt.Fprintf(fcode, "\n\tcase %v:", nprod)
+			fmt.Fprintf(fcode, "\n\t\t%sDollar = %sS[%spt-%v:%spt+1]", prefix, prefix, prefix, mem-1, prefix)
 			cpyact(curprod, mem)
 
 			// action within rule...
@@ -1345,7 +1346,7 @@ loop:
 				ungetrune(finput, c)
 				continue loop
 			}
-			fmt.Fprintf(fcode, "%sS[%spt-%v]", prefix, prefix, max-j-1)
+			fmt.Fprintf(fcode, "%sDollar[%v]", prefix, j)
 
 			// put out the proper tag
 			if ntypes != 0 {
@@ -3264,6 +3265,7 @@ func $$Parse($$lex $$Lexer) int {
 	var $$n int
 	var $$lval $$SymType
 	var $$VAL $$SymType
+	var $$Dollar []$$SymType
 	$$S := make([]$$SymType, $$MaxDepth)
 
 	Nerrs := 0   /* number of errors */