In CLs 122575 and 123177 the cgo tool started explicitly looking up
typedefs. When there are two Go files using import "C", and the first
one has an incomplete typedef and the second one has a complete
version of the same typedef, then we will now record a version of the
first typedef which will not match the recorded version of the second
typedef, producing an "inconsistent definitions" error. Fix this by
silently merging incomplete typedefs with complete ones.

Fixes #26430

Change-Id: I9e629228783b866dd29b5c3a31acd48f6e410a2d
Reviewed-on: https://go-review.googlesource.com/124575
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>