HLSL: add optional position.Y inversion

Adds command line options:

   --invert-y
   --iy

(synonyms) which invert position.Y on vertex shader output.  Handles these cases:

* Direct single variable return
* Member of direct returned struct
* Single variable output parameter
* Member of struct output parameter

API:

    // Enables position.Y output negation in vertex shader
    void TShader::setInvertY(bool invert);

Fixes #1173

glslang/Public/ShaderLang.h

@@ -371,6 +371,7 @@ public:
     void setResourceSetBinding(const std::vector<std::string>& base);
     void setAutoMapBindings(bool map);
     void setAutoMapLocations(bool map);
+    void setInvertY(bool invert);
     void setHlslIoMapping(bool hlslIoMap);
     void setFlattenUniformArrays(bool flatten);
     void setNoStorageFormat(bool useUnknownFormat);