D3D12 doesn't have native pipeline constants feature.
This is done by using #define. Add some new tests to make
sure these define approaches work as expected.
Also makes duplicate pipeline constant entries an invalid
case.
Bug: dawn:1137
Change-Id: Iefed44a749625b535bbafbb39f42699f0b42e06a
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/68860
Reviewed-by: Austin Eng <enga@chromium.org>
Commit-Queue: Shrek Shao <shrekshao@google.com>