BaseTools/Expression: Use 2nd passes on PCD values

BaseTools/Expression: Use 2nd passes on PCD values

Use 2 passes when evaluating PCD values to discover
all the LABEL() operators and compute the byte offset
of each LABEL().  The 2nd pass then has the information
to replace the OFFSET_OF() operator with the computed
byte offset.  The 2 passes allows OFFSET_OF() to be used
before a LABEL() is declared.

fixes:https://bugzilla.tianocore.org/show_bug.cgi?id=880
Cc: Liming Gao <liming.gao@intel.com>
Cc: Yonghong Zhu <yonghong.zhu@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Michael D Kinney <michael.d.kinney@intel.com>
Reviewed-by: Yonghong Zhu <yonghong.zhu@intel.com>