BaseTools/UPT: Fix an issue of storing wrong PCD into XML file.

Fix an issue of storing wrong PCD into XML file to only store PcdEx for AsBuilt sections

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Hess Chen <hesheng.chen@intel.com>
Reviewed-by: Yingke Liu <yingke.d.liu@Intel.com>


git-svn-id: svn://svn.code.sf.net/p/edk2/code/trunk/edk2/BaseTools@16426 6f19259b-4bc3-4df7-8a09-765794883524