HLSL: Handle "fake" entry points, by undoing their built-in variable declarations.

Test/hlsl.multiEntry.vert

@@ -0,0 +1,11 @@
+Buffer<float4> Position;
+
+float4 FakeEntrypoint(uint Index : SV_VERTEXID) : SV_POSITION
+{ 
+    return Position.Load(Index);
+}
+
+float4 RealEntrypoint(uint Index : SV_VERTEXID) : SV_POSITION
+{ 
+    return FakeEntrypoint(Index);
+}