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.cpp

@@ -80,6 +80,8 @@ namespace glslang {
             return EatPatchConstantFunc;
         else if (lowername == "unroll")
             return EatUnroll;
+        else if (lowername == "loop")
+            return EatLoop;
         else
             return EatNone;
     }
@@ -107,4 +109,10 @@ namespace glslang {
         return (entry == attributes.end()) ? nullptr : entry->second;
     }
 
+    // True if entry exists in map (even if value is nullptr)
+    bool TAttributeMap::contains(TAttributeType attr) const
+    {
+        return attributes.find(attr) != attributes.end();
+    }
+
 } // end namespace glslang