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/+/2372664Reviewed-by: Charlie Lao <cclao@google.com>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Commit-Queue: Tim Van Patten <timvp@google.com>