Vulkan: Convert ProgramExecutableVk::mDescriptorSets to std::array Currently ProgramExecutableVk::mDescriptorSets is a vector, but it's size is bound so it can be a std::array (DescriptorSetLayoutArray). To ensure the size grows correctly in the future, the various descriptor set indexes are also being converted from independent constexpr uint32_ts into the enum DescriptorSetIndex. Bug: angleproject:4898 Test: CQ Change-Id: I7ae8ff3455bcfb61e24b73bd16cc3f8cf9873087 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2372664 Reviewed-by: Charlie Lao <cclao@google.com> Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org> Commit-Queue: Tim Van Patten <timvp@google.com>