Add unit test for SV_ShadingRate output from mesh shader. (#2600)

tools/clang/test/HLSLFileCheck/shader_targets/mesh/mesh-shadingrate.hlsl

@@ -0,0 +1,29 @@
+// RUN: %dxc -E main -T ms_6_5 %s | FileCheck %s
+
+// CHECK: dx.op.storePrimitiveOutput.i32(i32 172,
+
+struct MSvert {
+  float4 pos : SV_Position;
+};
+
+struct MSprim
+{
+    uint id : SV_PrimitiveID;
+    uint shadingRate : SV_ShadingRate;
+};
+
+[outputtopology("triangle")]
+[numthreads(1,1,1)]
+void main(
+    in uint3 groupID : SV_GroupID, in uint3 threadInGroup : SV_GroupThreadID,
+    out vertices MSvert verts[3],
+    out primitives MSprim prims[1],
+    out indices uint3 idx[1])
+{
+   SetMeshOutputCounts(3, 1);
+   verts[0].pos = float4( 0.0f,   0.25f, 1.0f, 1.0f);
+   verts[1].pos = float4( 0.25f, -0.25f, 1.0f, 1.0f);
+   verts[2].pos = float4(-0.25f, -0.25f, 1.0f, 1.0f);
+   prims[0].shadingRate = 0x5; // D3D12_SHADING_RATE_2X2
+   idx[0]   = uint3(0,1,2);
+}