vkd3d-shader: Create a separate variable for input varyings.

Signed-off-by: Zebediah Figura <zfigura@codeweavers.com>
Signed-off-by: Matteo Bruni <mbruni@codeweavers.com>
Signed-off-by: Henri Verbeet <hverbeet@codeweavers.com>
Signed-off-by: Alexandre Julliard <julliard@winehq.org>

tests/hlsl-invalid.shader_test

@@ -135,3 +135,14 @@ float4 main() : sv_target
     const float4 x;
     return x;
 }
+
+[pixel shader fail]
+struct input
+{
+    float4 a;
+};
+
+float4 main(struct input i) : sv_target
+{
+    return i.a;
+}