Some C types are declared as pointers, but C code
stores non-pointers in them.  When the Go garbage
collector sees such a pointer, it gets unhappy.

Instead, for these types represent them on the Go
side with uintptr.

We need this change to handle Apple's CoreFoundation
CF*Ref types. Users of these types might need to
update their code like we do in root_cgo_darwin.go.
The only change that is required under normal
circumstances is converting some nils to 0.
A go fix module is provided to help.

Fixes #21897

RELNOTE=yes

Change-Id: I9716cfb255dc918792625f42952aa171cd31ec1b
Reviewed-on: https://go-review.googlesource.com/66332
Run-TryBot: Keith Randall <khr@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>