BaseTools: Fix the bug 'DSC DEFAULT' in report wrongly use FDF value

BaseTools: Fix the bug 'DSC DEFAULT' in report wrongly use FDF value

current the PCD value in DSC file may be override by FDF file, then it
cause the 'DSC DEFAULT' in build report wrongly display the FDF value
but not the DSC file's value.
This patch add a attribute DscDefaultValue for PcdClassObject to save
the actual DSC file's PCD value and use this value to display for 'DSC
DEFAULT'.

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