MdeModulePkg:Restore the HiiValue for string opcode

MdeModulePkg:Restore the HiiValue for string opcode

String opcode has HiiValue and BufferValue,BufferValue for string content,
HiiValue for StringId.Function GetQuestionValue only update the
BufferValue(string content),but the StringId and string content
do not match,now fix this issue.

Contributed-under: TianoCore Contribution Agreement 1.0
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>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@19750 6f19259b-4bc3-4df7-8a09-765794883524