[cfi] Move all vtables to .data.rel.ro in cross-DSO mode

[eugenis]

Summary

All call targets (which for virtual calls are vtable address points)
must be located above __cfi_check.
https://clang.llvm.org/docs/ControlFlowIntegrityDesign.html#cfi-shadow

[eugenis]

We've decided to scrap the __cfi_check order requirement.

lib/Transforms/IPO/LowerTypeTests.cpp

GlobalInits.push_back(
         ConstantAggregateZero::get(ArrayType::get(Int8Ty, Padding)));
   }
   if (!GlobalInits.empty())
     GlobalInits.pop_back();
   Constant *NewInit = ConstantStruct::getAnon(M.getContext(), GlobalInits);
   auto *CombinedGlobal =
       new GlobalVariable(M, NewInit->getType(), /*isConstant=*/true,
                          GlobalValue::PrivateLinkage, NewInit);
+  // Cross-DSO CFI has a requirement that CFI targets are located at higher
+  // addresses than __cfi_check. This is broken in the rare case when a virtual
+  // table is emitted in .rodata (which can only happen for virtual classes w/o
+  // virtual methods and -fno-rtti). Move them to .data.rel.ro with the rest.
+  if (ObjectFormat == Triple::ELF && M.getModuleFlag("Cross-DSO CFI"))
+    CombinedGlobal->setSection(".data.rel.ro");
 
   StructType *NewTy = cast<StructType>(NewInit->getType());
   const StructLayout *CombinedGlobalLayout = DL.getStructLayout(NewTy);
 
   // Compute the offsets of the original globals within the new global.
   DenseMap<GlobalTypeMember *, uint64_t> GlobalLayout;
   for (unsigned I = 0; I != Globals.size(); ++I)
     // Multiply by 2 to account for padding elements.

test/Transforms/LowerTypeTests/cross-dso-ro.ll

+; Check that in cross-DSO mode all constants with !type go to .data.rel.ro.
+; RUN: opt -mtriple i686-linux -S -lowertypetests < %s | FileCheck %s
+; RUN: opt -mtriple i686-linux -S -passes=lowertypetests < %s | FileCheck %s
+
+target datalayout = "e-p:32:32"
+
+; CHECK: @0 = private constant { i32 } { i32 1 }, section ".data.rel.ro"
+; CHECK: @a = alias i32, getelementptr inbounds ({ i32 }, { i32 }* @0, i32 0, i32 0
+@a = constant i32 1, !type !0
+
+declare i1 @llvm.type.test(i8* %ptr, metadata %bitset) nounwind readnone
+
+; CHECK-LABEL: @foo(
+define i1 @foo() {
+  ; CHECK: ret i1 true
+  %x = call i1 @llvm.type.test(i8* bitcast (i32* @a to i8*), metadata !"typeid1")
+  ret i1 %x
+}
+
+!llvm.module.flags = !{!1}
+
+!0 = !{i32 0, !"typeid1"}
+!1 = !{i32 4, !"Cross-DSO CFI", i32 1}