reflect: fix unsafe conversions reported by -d=checkptr
The code for generating gcdata was (technically) unsafe. It was also
rather repetitive. This CL refactors it a bit and abstracts use of
gcdata into a helper gcSlice method.
Updates #34972.
Change-Id: Ie86d7822eafe263f1d3d150eedf0ec66be1ec85d
Reviewed-on: https://go-review.googlesource.com/c/go/+/202582
Run-TryBot: Matthew Dempsky <mdempsky@google.com> Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com> Reviewed-by: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>