go/internal/gccgoimporter: update for gofrontend export data changes

This recognizes new features that the gofrontend has started emitting
in the export data to support cross-package inlinable functions.

This is a port of CL 180677 and 180758 from the gofrontend repo.

Change-Id: I48af6e71f9d8b04ba874ea0c204d39d1d461f8ad
Reviewed-on: https://go-review.googlesource.com/c/go/+/181118


Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>