compiler: better error for unexpected digit

A digit character is not invalid in general, but it's not permitted
at the start of an identifier.  Report a better error message.
The test case is issue11359.go in the source repo.

Change-Id: If8c55aba710419715a7932067023102f61efc742
Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/278174
Trust: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>
Reviewed-by: Cherry Zhang <cherryyz@google.com>
1 file changed