BaseTools: INF should use latest Pcd value instead of default value

BaseTools: INF should use latest Pcd value instead of default value

This patch is a bug fix about FeatureFlagExpression in INF file:
 INF [Source] section now unconditionally use Pcd default value in DEC
 when handling FeatureFlagExpression, it is wrong.
 If a Pcd value has been set in the DSC file, we should use latest
 value in DSC instead of default value.

Cc: Bob Feng <bob.c.feng@intel.com>
Cc: Liming Gao <gaoliming@byosoft.com.cn>
Signed-off-by: Yi Li <yi1.li@intel.com>
Reviewed-by: Bob Feng <bob.c.feng@intel.com>