cmd/compile: lazily initialize litbuf

Instead of eagerly creating strings like "literal 2.01" for every
lexed number in case we need to mention it in an error message, defer
this work to (*parser).syntax_error.

name      old allocs/op  new allocs/op  delta
Template      482k ± 0%      482k ± 0%  -0.12%   (p=0.000 n=9+10)
GoTypes      1.35M ± 0%     1.35M ± 0%  -0.04%  (p=0.015 n=10+10)
Compiler     5.45M ± 0%     5.44M ± 0%  -0.12%    (p=0.000 n=9+8)

Change-Id: I333b3c80e583864914412fb38f8c0b7f1d8c8821
Reviewed-on: https://go-review.googlesource.com/22480
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/cmd/compile/internal/gc/lex.go

@@ -755,7 +755,7 @@ func (l *lexer) number(c rune) {
 	}
 
 done:
-	litbuf = "literal " + str
+	litbuf = "" // lazily initialized in (*parser).syntax_error
 	l.nlsemi = true
 	l.tok = LLITERAL
 }

src/cmd/compile/internal/gc/parser.go

@@ -102,6 +102,9 @@ func (p *parser) syntax_error(msg string) {
 			tok = "name"
 		}
 	case LLITERAL:
+		if litbuf == "" {
+			litbuf = "literal " + lexbuf.String()
+		}
 		tok = litbuf
 	case LOPER:
 		tok = goopnames[p.op]