[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 01e072db5d26c224dfbe7763a5b94ab23c163983)
    Reviewed-on: https://go-review.googlesource.com/c/154299
    Run-TryBot: Filippo Valsorda <filippo@golang.org>
    Reviewed-by: Bryan C. Mills <bcmills@google.com>