Summary:
This patch sets up the infrastructure for
1. Associate MCSymbolXCOFF with an MCSectionXCOFF when it could not
get implicitly associated.
2. Generate undefined symbols. The patch itself generates undefined symbol
for external function call only. Generate undefined symbol for external
global variable and external function descriptors will be handled in
separate patch(s) after this is land.
Differential Revision: https://reviews.llvm.org/D70443
jasonliu