refactor/rename: fail cleanly when attempting to rename cgo files

Previous implementation will overwrite files the import "C" with the
cgo preprocessing and renaming. Rename will now emit an error when
rename must edit files that import "C". Will also emit more useful
error when using -offset in a "C" importing file.

Fixes golang/go#17839

Change-Id: I072100b22197ec145b56d727feca58be7529e359
Reviewed-on: https://go-review.googlesource.com/45930
Reviewed-by: Alan Donovan <adonovan@google.com>
3 files changed