BaseTools:Not miss the full assign value of FixedAtBuild structure PCD

BaseTools:Not miss the full assign value of FixedAtBuild structure PCD

For structure PCD, if it is a FixedAtBuild PCD, the full assign value in
dsc file should not be missed when updating the structure PCD value.

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: ZhiqiangX Zhao <zhiqiangx.zhao@intel.com>
Cc: Liming Gao <liming.gao@intel.com>
Cc: Yonghong Zhu <yonghong.zhu@intel.com>
Cc: Bob Feng <bob.c.feng@intel.com>
Reviewed-by: Bob Feng <bob.c.feng@intel.com>