cmd/compile: lazily initialize litbuf

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>