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

lib/MC/MCObjectFileInfo.cpp

@@ -819,6 +819,11 @@ void MCObjectFileInfo::initCOFFMCObjectFileInfo(const Triple &T) {
   SXDataSection = Ctx->getCOFFSection(".sxdata", COFF::IMAGE_SCN_LNK_INFO,
                                       SectionKind::getMetadata());
 
+  GFIDsSection = Ctx->getCOFFSection(".gfids$y",
+                                     COFF::IMAGE_SCN_CNT_INITIALIZED_DATA |
+                                         COFF::IMAGE_SCN_MEM_READ,
+                                     SectionKind::getMetadata());
+
   TLSDataSection = Ctx->getCOFFSection(
       ".tls$", COFF::IMAGE_SCN_CNT_INITIALIZED_DATA | COFF::IMAGE_SCN_MEM_READ |
                    COFF::IMAGE_SCN_MEM_WRITE,