Emit Function IDs table for Control Flow Guard

Emit Function IDs table for Control Flow Guard

Adds option /guard:cf to clang-cl and -cfguard to cc1 to emit function IDs
of functions that have their address taken into a section named .gfids$y for
compatibility with Microsoft's Control Flow Guard feature.

Differential Revision: https://reviews.llvm.org/D40531

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@322005 91177308-0d34-0410-b5e6-96231b3b80d8