BaseTools: Fix a bug for Hii Pcd override

BaseTools: Fix a bug for Hii Pcd override

BZ: https://bugzilla.tianocore.org/show_bug.cgi?id=2157

Hii Pcd links to a efi variable. The Variable
default value is evaluated by related Hii Pcds
setting. If multiple Hii Pcds links to one variable,
and the offset overlap, the later Hii Pcds setting
should be effective. There is a tool bug that is if
the Pcds are in different dsc file which are included
into the platform dsc file, build tool does not get
the Pcds relative position correctly. That means
build tool does not know which Pcd is the later one. As
the result, the variable default value will be incorrect.

This patch is to fix this bug.

Cc: Liming Gao <liming.gao@intel.com>
Signed-off-by: Bob Feng <bob.c.feng@intel.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>