typecheckcomplit nils out node's type, upon finding new errors.
This hides new errors in children's node as well as the type info
of current node. This change fixes that.

Fixes #17645.

Change-Id: Ib473291f31c7e8fa0307cb1d494e0c112ddd3583
Reviewed-on: https://go-review.googlesource.com/32324Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>