BaseTools: Fix a bug --pcd option enable and use the pcd in expression

the case is:
in the DSC:
[PcdsFixedAtBuild.common]
 TokenSpaceGuid.TestFixedPcd|0xFFEAA000

[PcdsDynamicExDefault.common.DEFAULT]
!if TokenSpaceGuid.PcdFlag == TRUE
TokenSpaceGuid.PcdTest|TokenSpaceGuid.TestFixedPcd
!endif

Then build with --pcd TokenSpaceGuid.PcdFlag=TRUE, it report failure,
but if we build without this --pcd option, it could build success.
we found when the --pcd is enabled, the fixedatbuild pcds are not be
collected into expression to calculate.

Fixes: https://bugzilla.tianocore.org/show_bug.cgi?id=1256
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Yonghong Zhu <yonghong.zhu@intel.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>

BaseTools/Source/Python/Workspace/DscBuildData.py

@@ -2946,4 +2946,5 @@ class DscBuildData(PlatformBuildClassObject):
                 ModuleData = self._Bdb[ModuleFile, self._Arch, self._Target, self._Toolchain]
                 PkgSet.update(ModuleData.Packages)
             self._DecPcds, self._GuidDict = GetDeclaredPcd(self, self._Bdb, self._Arch, self._Target, self._Toolchain, PkgSet)
+            self._GuidDict.update(GlobalData.gPlatformPcds)
         return self._DecPcds