Use a combination of follow- and stop-token lists and nesting levels
to better synchronize parser after a syntax error.

Fixes #13319.

Change-Id: I9592e0b5b3ba782fb9f9315fea16163328e204f7
Reviewed-on: https://go-review.googlesource.com/17080Reviewed-by: Chris Manghane <cmang@golang.org>