When on 1.15+, enable TypecheckCgo. This improves cgo support
significantly, but we'll still have trouble with newly-referenced C
identifiers and changes to the magic comment.
Updates golang/go#35721.
Change-Id: I44dc95ce2d91d552e1e66e3722dc4230ab59fedd
Reviewed-on: https://go-review.googlesource.com/c/tools/+/229779
Run-TryBot: Heschi Kreinick <heschi@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rebecca Stambler <rstambler@golang.org>
