[Backtracing] Add _Backtracing library to the build.

Adds a new swift_Backtracing library, with a corresponding _Backtracing
module, to the build.  Also add some tests.

This is not public API at this point, but will be used by the external
backtracing program, `swift-backtrace`.

rdar://104336548

Author: al45tair

stdlib/public/Backtracing/Backtrace.swift

@@ -0,0 +1,53 @@
+#===--- CMakeLists.txt - Backtracing support library -----------------------===#
+#
+# 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
+#
+#===------------------------------------------------------------------------===#
+
+set(swift_backtracing_link_libraries
+  swiftCore
+)
+
+set(BACKTRACING_SOURCES
+  Backtrace.swift
+  BacktraceFormatter.swift
+  Context.swift
+  CoreSymbolication.swift
+  FramePointerUnwinder.swift
+  MemoryReader.swift
+  Registers.swift
+  SymbolicatedBacktrace.swift
+  Utils.swift
+
+  get-cpu-context.S
+)
+
+add_swift_target_library(swift_Backtracing ${SWIFT_STDLIB_LIBRARY_BUILD_TYPES} IS_STDLIB
+  ${BACKTRACING_SOURCES}
+
+  SWIFT_MODULE_DEPENDS_IOS Darwin _Concurrency
+  SWIFT_MODULE_DEPENDS_OSX Darwin _Concurrency
+  SWIFT_MODULE_DEPENDS_TVOS Darwin _Concurrency
+  SWIFT_MODULE_DEPENDS_WATCHOS Darwin _Concurrency
+  SWIFT_MODULE_DEPENDS_MACCATALYST Darwin _Concurrency
+  SWIFT_MODULE_DEPENDS_LINUX Glibc _Concurrency
+  SWIFT_MODULE_DEPENDS_WINDOWS CRT _Concurrency
+
+  LINK_LIBRARIES ${swift_backtracing_link_libraries}
+
+  SWIFT_COMPILE_FLAGS
+    ${SWIFT_STANDARD_LIBRARY_SWIFT_FLAGS}
+    -parse-stdlib
+
+  LINK_FLAGS
+    ${SWIFT_RUNTIME_SWIFT_LINK_FLAGS}
+
+  INSTALL_IN_COMPONENT stdlib
+  MACCATALYST_BUILD_FLAVOR "zippered"
+)