HLSL: add standard sample position return form of GetSamplePosition method

Multisample textures support a GetSamplePosition() method intended to query
positions given a sample index.  This cannot be truly implemented in SPIR-V,
but #753 requested returning standard positions for the 1..16 cases, which
this PR adds.  Anything besides that returns (0,0).  If the standard positions
are not used, this will be wrong.

This should be revisited when there is a real query available.

hlsl/hlslParseHelper.h

@@ -273,6 +273,9 @@ protected:
     // Test method names
     bool isStructBufferMethod(const TString& name) const;
 
+    // Return standard sample position array
+    TIntermConstantUnion* getSamplePosArray(int count);
+
     TType* getStructBufferContentType(const TType& type) const;
     bool isStructBufferType(const TType& type) const { return getStructBufferContentType(type) != nullptr; }
     TIntermTyped* indexStructBufferContent(const TSourceLoc& loc, TIntermTyped* buffer) const;