go/types: rename UsesCgo to go115UsesCgo

This API and functionality was added late in the Go 1.15 release
cycle, and use within gopls has revealed some shortcomings. It's
possible (but not decided) that we'll want a different API long-term,
so for now this CL renames UsesCgo to a non-exported name to avoid
long-term commitment under the Go 1 compat guarantee.

Updates #16623.
Updates #39072.

Change-Id: I04bc0c161a84adebe43e926df5df406bc794c3db
Reviewed-on: https://go-review.googlesource.com/c/go/+/237417


Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Heschi Kreinick <heschi@google.com>