[Backtracing][Linux] Add missing CrashInfo.h header.

This was added to a later PR, but not to this one, though we need
it here.

rdar://110261430

Author: al45tair

include/swift/Runtime/CrashInfo.h

@@ -0,0 +1,70 @@
+//===--- CrashInfo.h - Swift Backtracing Crash Information ------*- 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 the CrashInfo type that holds information about why the program
+//  crashed.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef SWIFT_CRASHINFO_H
+#define SWIFT_CRASHINFO_H
+
+#include <inttypes.h>
+
+#ifdef __cplusplus
+namespace swift {
+namespace runtime {
+namespace backtrace {
+extern "C" {
+#endif
+
+struct CrashInfo {
+  uint64_t crashing_thread;
+  uint64_t signal;
+  uint64_t fault_address;
+
+#ifdef __APPLE__
+  uint64_t mctx;
+#elif defined(__linux__)
+  uint64_t thread_list;
+#endif
+};
+
+#ifdef __linux__
+
+struct memserver_req {
+  uint64_t addr;
+  uint64_t len;
+};
+
+struct memserver_resp {
+  uint64_t addr;
+  int64_t  len;
+  /* Then len bytes of data */
+};
+
+struct thread {
+  uint64_t next;
+  int64_t  tid;
+  uint64_t uctx;
+};
+
+#endif
+
+#ifdef __cplusplus
+} // extern "C"
+} // namespace backtrace
+} // namespace runtime
+} // namespace swift
+#endif
+
+#endif // SWIFT_CRASHINFO_H