[Backtracing] Needed to add some code to this header for the runtime.

In order for the runtime PR to work as a separate PR, it does need a little
bit of code in the `_SwiftBacktracing.h` header in SwiftShims.

rdar://105391747

Author: al45tair

stdlib/public/SwiftShims/swift/shims/_SwiftBacktracing.h

@@ -0,0 +1,39 @@
+//===--- _SwiftBacktracing.h - Swift Backtracing Support --------*- C++ -*-===//
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2023 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
+//
+//===----------------------------------------------------------------------===//
+//
+//  Defines types and support functions for the Swift backtracing code.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef SWIFT_BACKTRACING_H
+#define SWIFT_BACKTRACING_H
+
+#include <inttypes.h>
+
+#ifdef __cplusplus
+namespace swift {
+extern "C" {
+#endif
+
+struct CrashInfo {
+  uint64_t crashing_thread;
+  uint64_t signal;
+  uint64_t fault_address;
+  uint64_t mctx;
+};
+
+#ifdef __cplusplus
+} // extern "C"
+} // namespace swift
+#endif
+
+#endif // SWIFT_BACKTRACING_H