Currently readType simultaneously constructs a type graph and resolves
the sizes of the types. However, these two operations are
fundamentally at odds: the order we parse a cyclic structure in may be
different than the order we need to resolve type sizes in. As a
result, it's possible that when readType attempts to resolve the size
of a typedef, it may dereference a nil Type field of another typedef
retrieved from the type cache that's only partially constructed.

To fix this, we delay resolving typedef sizes until the end of the
readType recursion, when the full type graph is constructed.

Fixes #13039.

Change-Id: I9889af37fb3be5437995030fdd61e45871319d07
Reviewed-on: https://go-review.googlesource.com/18459Reviewed-by: Russ Cox <rsc@golang.org>
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>