Pipeline overridable constants: D3D12 backend

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>

src/tests/unittests/validation/OverridableConstantsValidationTests.cpp

@@ -105,12 +105,12 @@ TEST_F(ComputePipelineOverridableConstantsValidationTest, ConstantsIdentifierLoo
         TestCreatePipeline(constants);
     }
     {
-        // Valid: set the same constant twice
+        // Error: set the same constant twice
         std::vector<wgpu::ConstantEntry> constants{
             {nullptr, "c0", 0},
             {nullptr, "c0", 1},
         };
-        TestCreatePipeline(constants);
+        ASSERT_DEVICE_ERROR(TestCreatePipeline(constants));
     }
     {
         // Valid: find by constant numeric id
@@ -158,12 +158,12 @@ TEST_F(ComputePipelineOverridableConstantsValidationTest, UninitializedConstants
         TestCreatePipeline(constants);
     }
     {
-        // Valid: all constants initialized (with duplicate initializations)
+        // Error: duplicate initializations
         std::vector<wgpu::ConstantEntry> constants{
             {nullptr, "c0", false}, {nullptr, "c2", 1}, {nullptr, "c5", 1},
             {nullptr, "c8", 1},     {nullptr, "c2", 2},
         };
-        TestCreatePipeline(constants);
+        ASSERT_DEVICE_ERROR(TestCreatePipeline(constants));
     }
 }