If we're running coverage on a package using cgo, we need to
apply both cmd/cover and cmd/cgo as source transformers.
To date we've applied cgo, then cover.

Cover is very sensitive to the exact character position of
expressions in its input, though, and cgo is not, so swap
them, applying first cover and then cgo.

The only drawback here is that coverage formerly applied
to SWIG-generated cgo files, and now it does not.
I am not convinced anyone depended critically on that,
and probably the later analysis with go tool cover would
have tried to parse the original .swig file as a Go file and
gotten very confused.

Fixes #8726.
Fixes #9212.
Fixes #9479.

Change-Id: I777c8b64f7726cb117d59e03073954abc6dfa34d
Reviewed-on: https://go-review.googlesource.com/77155Reviewed-by: Ian Lance Taylor <iant@golang.org>