go/internal/gccgoimporter: fix bug reading V1 export data

Fix a bug in the reading of elderly export data. In such export data
when reading type information it's possible to encounter a named type N1
defined as a typedef of some other named type N2 at a point when the
underying type of N1 has not yet been finalized. Handle this case by
generating a fixup, then process fixups at the end of parsing to
set the correct underlying type.

Fixes #29006.

Change-Id: I6a505c897bd95eb161ee04637bb6eebad9f20d52
Reviewed-on: https://go-review.googlesource.com/c/151997


Run-TryBot: Than McIntosh <thanm@google.com>
Reviewed-by: Robert Griesemer <gri@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>