C: add Cosmos Nicolaou (Google CLA)

R=golang-dev, bradfitz
CC=golang-dev
https://codereview.appspot.com/7314103
go/parser: cleanups following CL 7307085

- use the new AllErrors flag where appropriate
- unless AllErrors is set, eliminate spurious
  errors before they are added to the errors list
  (it turns out that reporting spurious errors always
  leads to too many uninformative errors after all)

R=golang-dev, bradfitz
CC=golang-dev
https://codereview.appspot.com/7323065