HLSL: implement [unroll] and [loop] attributes

This adds infrastructure suitable for any front end to create SPIR-V loop
control flags.  The only current front end doing so is HLSL.

[unroll] turns into spv::LoopControlUnrollMask
[loop] turns into spv::LoopControlDontUnrollMask
no specification means spv::LoopControlMaskNone

hlsl/hlslParseHelper.h

@@ -192,6 +192,9 @@ public:
     bool handleOutputGeometry(const TSourceLoc&, const TLayoutGeometry& geometry);
     bool handleInputGeometry(const TSourceLoc&, const TLayoutGeometry& geometry);
 
+    // Determine loop control from attributes
+    TLoopControl handleLoopControl(const TAttributeMap& attributes) const;
+
     // Potentially rename shader entry point function
     void renameShaderFunction(const TString*& name) const;