[Runtime] Add a function pointer for intercepting swift_willThrow calls.

rdar://problem/53400364

stdlib/public/runtime/ErrorObjectCommon.cpp

@@ -0,0 +1,38 @@
+//===--- ErrorObjectCommon.cpp - Recoverable error object -----------------===//
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2014 - 2019 Apple Inc. and the Swift project authors
+// Licensed under Apache License v2.0 with Runtime Library Exception
+//
+// See https://swift.org/LICENSE.txt for license information
+// See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
+//
+//===----------------------------------------------------------------------===//
+//
+// This implements the parts of the standard Error protocol type which are
+// shared between the ObjC-interoperable implementation and the native
+// implementation. The parts specific to each implementation can be found in
+// ErrorObject.mm (for the ObjC-interoperable parts) and ErrorObjectNative.cpp.
+//
+//===----------------------------------------------------------------------===//
+
+#include "swift/Runtime/Concurrent.h"
+#include "swift/Runtime/Config.h"
+#include "ErrorObject.h"
+#include "ErrorObjectTestSupport.h"
+
+using namespace swift;
+
+void (*swift::_swift_willThrow)(SwiftError *error);
+
+/// Breakpoint hook for debuggers, and calls _swift_willThrow if set.
+SWIFT_CC(swift) void
+swift::swift_willThrow(SWIFT_CONTEXT void *unused,
+                       SWIFT_ERROR_RESULT SwiftError **error) {
+  // Cheap check to bail out early, since we expect there to be no callbacks
+  // the vast majority of the time.
+  if (SWIFT_LIKELY(!_swift_willThrow))
+    return;
+  _swift_willThrow(*error);
+}