[Backtracing] Rename _Backtracing to Runtime.

Move the backtracing code into a new Runtime module.  This means renaming
the Swift Runtime's CMake target because otherwise there will be a name
clash.

rdar://124913332

stdlib/public/RuntimeModule/Win32Extras.cpp

@@ -0,0 +1,57 @@
+//===--- Win32Extras.cpp - Windows support functions ------------*- 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 some extra functions that aren't available in the OS or C library
+//  on Windows.
+//
+//===----------------------------------------------------------------------===//
+
+#ifdef _WIN32
+
+#include <windows.h>
+
+#include "modules/OS/Libc.h"
+
+extern "C" ssize_t pread(int fd, void *buf, size_t nbyte, off_t offset) {
+  HANDLE hFile = _get_osfhandle(fd);
+  OVERLAPPED ovl = {0};
+  DWORD dwBytesRead = 0;
+
+  ovl.Offset = (DWORD)offset;
+  ovl.OffsetHigh = (DWORD)(offset >> 32);
+
+  if (!ReadFile(hFile, buf, (DWORD)count, &dwBytesRead, &ovl)) {
+    errno = EIO;
+    return -1;
+  }
+
+  return dwBytesRead;
+}
+
+extern "C" ssize_t pwrite(int fd, const void *buf, size_t nbyte, off_t offset) {
+  HANDLE hFile = _get_osfhandle(fd);
+  OVERLAPPED ovl = {0};
+  DWORD dwBytesRead = 0;
+
+  ovl.Offset = (DWORD)offset;
+  ovl.OffsetHigh = (DWORD)(offset >> 32);
+
+  if (!WriteFile(hFile, buf, (DWORD)count, &dwBytesRead, &ovl)) {
+    errno = EIO;
+    return -1;
+  }
+
+  return dwBytesRead;
+}
+
+#endif // _WIN32
+