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/87898Reviewed-by: Matthew Dempsky <mdempsky@google.com>