GitLab

- only convert literal strings if there were no syntax errors
  (some of the conversion routines exit if there is an error)
- mark nodes for literals with syntax errors to avoid follow-on
  errors
- don't attempt to import packages whose path had syntax errors

Fixes #32133.

Change-Id: I1803ad48c65abfecf6f48ddff1e27eded5e282c5
Reviewed-on: https://go-review.googlesource.com/c/go/+/192437

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