cmd/compile: better syntax error handling for new parser

- better error messages
- better error recovery by advancing to "follow" token after error
- make sure that we make progress after all errors
- minor cleanups

Change-Id: Ie43b8b02799618d70dc8fc227fab3e4e9e0d8e3a
Reviewed-on: https://go-review.googlesource.com/16892
Run-TryBot: Robert Griesemer <gri@golang.org>
Reviewed-by: Chris Manghane <cmang@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

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

@@ -484,8 +484,7 @@ var nolocalimports int
 
 var lexbuf bytes.Buffer
 var strbuf bytes.Buffer
-
-var litbuf string
+var litbuf string // LLITERAL value for use in syntax error messages
 
 var Debug [256]int