go/internal/gcimporter: fail gracefully on export format skew

go/internal/gcimporter: fail gracefully on export format skew

Port of changes made to compiler in
https://go-review.googlesource.com/27814.

Correctly handle export format version 0 (we only do this
in x/tools/gcimporter15 at the moment - this is a backport
of that code for struct fields).

Added tests for version handling and detection of corrupted
export data.

Fixes #16881.

Change-Id: I246553c689c89ef5c7fedd1e43717504c2838804
Reviewed-on: https://go-review.googlesource.com/27816
Reviewed-by: Matthew Dempsky <mdempsky@google.com>