BaseTools: Workspace - refactor a dict

change a dict to a set since we never examine the contents, just the keys.

Cc: Liming Gao <liming.gao@intel.com>
Cc: Yonghong Zhu <yonghong.zhu@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Jaben Carsey <jaben.carsey@intel.com>
Reviewed-by: Yonghong Zhu <yonghong.zhu@intel.com>

BaseTools/Source/Python/Workspace/BuildClassObject.py

@@ -136,7 +136,7 @@ class StructurePcd(PcdClassObject):
         self.PcdDefineLineNo = 0
         self.PkgPath = ""
         self.DefaultValueFromDec = ""
-        self.ValueChain = dict()
+        self.ValueChain = set()
         self.PcdFieldValueFromComm = collections.OrderedDict()
     def __repr__(self):
         return self.TypeName