Cross-DSO control flow integrity (Clang part).

Clang-side cross-DSO CFI.

* Adds a command line flag -f[no-]sanitize-cfi-cross-dso.
* Links a runtime library when enabled.
* Emits __cfi_slowpath calls is bitset test fails.
* Emits extra hash-based bitsets for external CFI checks.
* Sets a module flag to enable __cfi_check generation during LTO.

This mode does not yet support diagnostics.

llvm-svn: 255694

clang/lib/CodeGen/CodeGenFunction.h

@@ -3009,6 +3009,11 @@ public:
                  StringRef CheckName, ArrayRef<llvm::Constant *> StaticArgs,
                  ArrayRef<llvm::Value *> DynamicArgs);
 
+  /// \brief Emit a slow path cross-DSO CFI check which calls __cfi_slowpath
+  /// if Cond if false.
+  void EmitCfiSlowPathCheck(llvm::Value *Cond, llvm::ConstantInt *TypeId,
+                            llvm::Value *Ptr);
+
   /// \brief Create a basic block that will call the trap intrinsic, and emit a
   /// conditional branch to it, for the -ftrapv checks.
   void EmitTrapCheck(llvm::Value *Checked);