compiler: track //go:nointerface in export data

The magic //go:nointerface comment, used for field tracking, was only
implemented for conversions to interface types in the same package.
Record it in the export data, so that it works as expected for types
imported from a different package.

Change-Id: Ie450f47f5e54885b2db6cd690689ae5d3aa3c10d
Reviewed-on: https://go-review.googlesource.com/93075
Reviewed-by: Than McIntosh <thanm@google.com>
4 files changed