[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/modules/FixedLayout.h

@@ -0,0 +1,62 @@
+//===--- FixedLayout.h - Types whose layout must be fixed -------*- 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 whose in-memory layout must be fixed, which therefore have
+//  to be defined using C code.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef SWIFT_BACKTRACING_FIXED_LAYOUT_H
+#define SWIFT_BACKTRACING_FIXED_LAYOUT_H
+
+#include <stdint.h>
+
+#ifdef __cplusplus
+namespace swift {
+namespace runtime {
+namespace backtrace {
+#endif
+
+struct x86_64_gprs {
+  uint64_t _r[16];
+  uint64_t rflags;
+  uint16_t cs, fs, gs, _pad0;
+  uint64_t rip;
+  uint64_t valid;
+};
+
+struct i386_gprs {
+  uint32_t _r[8];
+  uint32_t eflags;
+  uint16_t segreg[6];
+  uint32_t eip;
+  uint32_t valid;
+};
+
+struct arm64_gprs {
+  uint64_t _x[32];
+  uint64_t pc;
+  uint64_t valid;
+};
+
+struct arm_gprs {
+  uint32_t _r[16];
+  uint32_t valid;
+};
+
+#ifdef __cplusplus
+} // namespace backtrace
+} // namespace runtime
+} // namespace swift
+#endif
+
+#endif // SWIFT_BACKTRACING_FIXED_LAYOUT_H