Add --hlsl-dx-position-w option

This reciprocates the w component of SV_Position in HLSL fragment shaders
to provide DirectX compatibility for HLSL shaders in Vulkan.

Fixes #2244

glslang/MachineIndependent/linkValidate.cpp

@@ -312,6 +312,7 @@ void TIntermediate::mergeModes(TInfoSink& infoSink, TIntermediate& unit)
     MERGE_TRUE(autoMapBindings);
     MERGE_TRUE(autoMapLocations);
     MERGE_TRUE(invertY);
+    MERGE_TRUE(dxPositionW);
     MERGE_TRUE(flattenUniformArrays);
     MERGE_TRUE(useUnknownFormat);
     MERGE_TRUE(hlslOffsets);