[Linker] Change module flag linking to be more extensible.

 - Instead of computing a bunch of buckets of different flag types, just do an
   incremental link resolving conflicts as they arise.

 - This also has the advantage of making the link result deterministic and not
   dependent on map iteration order.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@172634 91177308-0d34-0410-b5e6-96231b3b80d8