[release-branch.go1.11] cmd/cgo: don't cache bad pointer typedefs

The set of bad pointer typedefs changes as we see more typedefs, so
avoid looking in the cache when we find one.

Updates #29175
Fixes #29272

Change-Id: Idd82289bdd8628d11a983fa5ec96517e3a5bcbf1
Reviewed-on: https://go-review.googlesource.com/c/153597


Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
(cherry picked from commit 01e072db)
Reviewed-on: https://go-review.googlesource.com/c/154299


Run-TryBot: Filippo Valsorda <filippo@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>