BaseTools: Update Expression.py for string comparison and MACRO replace issue

1. Fix string comparison incorrect issue, we expected "ABC" is greater than
"AAD" since the second char 'B' is greater than 'A'.
2. fix MACRO not replace issue.

Cc: Liming Gao <liming.gao@intel.com>
Cc: Yonghong Zhu <yonghong.zhu@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Yunhua Feng <yunhuax.feng@intel.com>
Reviewed-by: Yonghong Zhu <yonghong.zhu@intel.com>
(cherry picked from commit 9efe8d6040)

BaseTools/Source/Python/Workspace/DscBuildData.py

@@ -808,7 +808,7 @@ class DscBuildData(PlatformBuildClassObject):
                 PkgSet.update(ModuleData.Packages)
 
             self._DecPcds, self._GuidDict = GetDeclaredPcd(self, self._Bdb, self._Arch, self._Target, self._Toolchain,PkgSet)
-
+            self._GuidDict.update(GlobalData.gPlatformPcds)
 
         if (PcdCName, TokenSpaceGuid) not in self._DecPcds:
             EdkLogger.error('build', PARSER_ERROR,