Vulkan: Move pipeline subset out of pipeline desc This way, the same pipeline desc can be used to query multiple caches with different subsets. This design requires the pipeline cache to be templated, while the previous design required the desc to be copied (so the subset bits would be different). Bug: angleproject:7369 Change-Id: Id3fd92e6f9b059069952ff092a5c867e79287604 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/3956940 Reviewed-by: Charlie Lao <cclao@google.com> Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org> Reviewed-by: Yuxin Hu <yuxinhu@google.com>