SPV: When passing structs of opaque types, flatten and pass the members instead.

This avoids either A) needing uniformConstant struct, or
B) initializing a struct with opaque members, as writing them is not
allowed.

gtests/Hlsl.FromFile.cpp

@@ -114,6 +114,7 @@ INSTANTIATE_TEST_CASE_P(
         {"hlsl.float1.frag", "PixelShaderFunction"},
         {"hlsl.float4.frag", "PixelShaderFunction"},
         {"hlsl.flatten.return.frag", "main"},
+        {"hlsl.flattenOpaque.frag", "main"},
         {"hlsl.forLoop.frag", "PixelShaderFunction"},
         {"hlsl.gather.array.dx10.frag", "main"},
         {"hlsl.gather.basic.dx10.frag", "main"},