BaseTool/VfrCompiler: Support Bit fields in EFI/Buffer VarStore

BaseTool/VfrCompiler: Support Bit fields in EFI/Buffer VarStore

REF: https://bugzilla.tianocore.org/show_bug.cgi?id=545

Enhance VfrCompiler to parse following case:
1. EFI/Buffer VarStore can contain bit fields in their structure.
2. For question Oneof/Checkbox/numeric, their storage can be
   bit fields of an EFI VarStore/Buffer VarStore.

Cc: Eric Dong <eric.dong@intel.com>
Cc: Liming Gao <liming.gao@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Dandan Bi <dandan.bi@intel.com>
Reviewed-by: Eric Dong <eric.dong@intel.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>