The new export format keeps track of all types that are exported.
If a type is seen that was exported before, only a reference to
that type is emitted. The importer maintains a list of all the
seen types and uses that list to resolve type references.

The existing compiler infrastructure's invariants assumes that
only named types are referred to before they are fully set up.
Referring to unnamed incomplete types causes problems. One of
the issues was #15548.

Added a new internal flag 'trackAllTypes' to enable/disable
this type tracking. With this change only named types are
tracked.

Verified that this fix also addresses #15548, even w/o the
prior fix for that issue (in fact that prior fix is turned
off if trackAllTypes is disabled because it's not needed).

The test for #15548 covers also this change.

For #15548.

Change-Id: Id0b3ff983629703d025a442823f99649fd728a56
Reviewed-on: https://go-review.googlesource.com/22839
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>