go/parser: improved error recovery after missing type
R=go1.11
This CL also introduces a new TODO in parser.go. To be
addressed in a separate CL to make this easier to review.
Also: Make parser's test harness easier to use by ignoring
auto-inserted (invisible) semicolons when computing error
positions. Adjusted testdata/commas.src accordingly.
Fixes #23434.
Change-Id: I050592d11d5f984f71185548394c000eea509205
Reviewed-on: https://go-review.googlesource.com/87898 Reviewed-by: Matthew Dempsky <mdempsky@google.com>