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/hlslAttributes.h

@@ -62,6 +62,7 @@ namespace glslang {
         EatPatchConstantFunc,
         EatPatchSize,
         EatUnroll,
+        EatLoop,
     };
 }
 
@@ -86,6 +87,9 @@ namespace glslang {
         // Const lookup: search for (but do not modify) the attribute in the map.
         const TIntermAggregate* operator[](TAttributeType) const;
 
+        // True if entry exists in map (even if value is nullptr)
+        bool contains(TAttributeType) const;
+
     protected:
         // Find an attribute enum given its name.
         static TAttributeType attributeFromName(const TString&);