Make BitSetT::Mask return a bit set This allows a Mask function to be implemented for BitSetArray, which is added in this CL. When using larger bitsets on 32-bit systems, the current Mask implementation prohibits its use. This is in preparation for a follow up change that uses Mask on such a bitset. Bug: angleproject:7369 Change-Id: If995d96ec1583a546f20bff277f3223e2f2490f5 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/4031493 Reviewed-by: Charlie Lao <cclao@google.com> Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>