HLSL: Non-functional: Move partial flattened access into symbol node.

Lays the groundwork for fixing issue #954.

Partial flattenings were previously tracked through a stack of active subsets
in the parse context, but full functionality needs AST nodes to represent
this across time, removing the need for parsecontext tracking.

hlsl/hlslParseHelper.h

@@ -204,10 +204,6 @@ public:
     // Potentially rename shader entry point function
     void renameShaderFunction(const TString*& name) const;
 
-    // Reset data for incrementally built referencing of flattened composite structures
-    void initFlattening() { flattenLevel.push_back(0); flattenOffset.push_back(0); }
-    void finalizeFlattening() { flattenLevel.pop_back(); flattenOffset.pop_back(); }
-
     // Share struct buffer deep types
     void shareStructBufferType(TType&);
 
@@ -242,7 +238,7 @@ protected:
 
     // Array and struct flattening
     TIntermTyped* flattenAccess(TIntermTyped* base, int member);
-    TIntermTyped* flattenAccess(int uniqueId, int member, const TType&);
+    TIntermTyped* flattenAccess(int uniqueId, int member, const TType&, int subset = -1);
     bool shouldFlatten(const TType&) const;
     bool wasFlattened(const TIntermTyped* node) const;
     bool wasFlattened(int id) const { return flattenMap.find(id) != flattenMap.end(); }
@@ -368,7 +364,6 @@ protected:
 
     TMap<int, TFlattenData> flattenMap;
     TVector<int> flattenLevel;  // nested postfix operator level for flattening
-    TVector<int> flattenOffset; // cumulative offset for flattening
 
     // IO-type map. Maps a pure symbol-table form of a structure-member list into
     // each of the (up to) three kinds of IO, as each as different allowed decorations,