Define angle::HashSet potentially using abseil Split change from CL using it for simplicity. Bug: angleproject:4889 Change-Id: Ie3e621dee624020581e9fd7679639332718948f6 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/3061439 Reviewed-by: Tim Van Patten <timvp@google.com> Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>