BaseTools/VfrCompile/Pccts: Make assignment operator not returning void

BaseTools/VfrCompile/Pccts: Make assignment operator not returning void

The assignment operators for class ANTLRTokenPtr return void in current
code.

This commit makes them return the reference to the object just like
primitive types do.

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>