BaseTools/VfrCompile: Add assignment operator definition for some classes

BaseTools/VfrCompile: Add assignment operator definition for some classes

For class that defines the copy constructor, it is better to add the
assignment operator definition as well.

This commit adds the definition for assignment operator for the classes
with the copy constructor defined.

Cc: Liming Gao <liming.gao@intel.com>
Cc: Yonghong Zhu <yonghong.zhu@intel.com>
Cc: Eric Dong <eric.dong@intel.com>
Cc: Dandan Bi <dandan.bi@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Hao Wu <hao.a.wu@intel.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>