HLSL: Allow expressions in attributes

For example:

[numthreads(2+2, 2*3, (1+FOO)*BAR)]

This will result in a thread count (4, 6, 8).

gtests/Hlsl.FromFile.cpp

@@ -86,6 +86,7 @@ INSTANTIATE_TEST_CASE_P(
         {"hlsl.array.multidim.frag", "main"},
         {"hlsl.assoc.frag", "PixelShaderFunction"},
         {"hlsl.attribute.frag", "PixelShaderFunction"},
+        {"hlsl.attribute.expression.comp", "main"},
         {"hlsl.basic.comp", "main"},
         {"hlsl.buffer.frag", "PixelShaderFunction"},
         {"hlsl.calculatelod.dx10.frag", "main"},