[runtimes] Run backdeployment CI on Github hosted runners (llvm#109984)

This removes the need for macOS nodes in Buildkite. It also moves to the
proper way of testing backdeployment, which is to actually run on the
target OS itself, instead of using packaged dylibs from previous OS
versions and trying to emulate backdeployment with DYLD_LIBRARY_PATH.

As a drive-by change, also fix a few back-deployment annotations that
were incorrect and add support for minor versions in the Lit feature
determining availability from the target triple.

Author: ldionne

.github/workflows/libcxx-build-and-test.yaml

@@ -193,17 +193,24 @@ jobs:
             **/crash_diagnostics/*
 
   macos:
-    runs-on: macos-14
     needs: [ stage1 ]
     strategy:
-      fail-fast: true
+      fail-fast: false
       matrix:
-        config: [
-          generic-cxx03,
-          generic-cxx23,
-          generic-modules,
-          apple-configuration
-        ]
+        include:
+        - config: generic-cxx03
+          os: macos-latest
+        - config: generic-cxx23
+          os: macos-latest
+        - config: generic-modules
+          os: macos-latest
+        - config: apple-configuration
+          os: macos-latest
+        - config: apple-system
+          os: macos-13
+        - config: apple-system-hardened
+          os: macos-13
+    runs-on: ${{ matrix.os }}
     steps:
       - uses: actions/checkout@v4
       - uses: maxim-lobanov/setup-xcode@v1

libcxx/test/configs/apple-libc++-backdeployment.cfg.in

@@ -0,0 +1,33 @@
+# Testing configuration for back-deployment against the system-provided libc++.
+#
+# Under this configuration, we compile and link all the test suite against the just-built
+# libc++, but we run against the system libc++.
+
+import os, site
+site.addsitedir(os.path.join('@LIBCXX_SOURCE_DIR@', 'utils'))
+import libcxx.test.params, libcxx.test.config, libcxx.test.dsl
+
+lit_config.load_config(config, '@CMAKE_CURRENT_BINARY_DIR@/cmake-bridge.cfg')
+
+config.substitutions.append(('%{flags}',
+    '-isysroot {}'.format('@CMAKE_OSX_SYSROOT@') if '@CMAKE_OSX_SYSROOT@' else ''
+))
+config.substitutions.append(('%{compile_flags}',
+    '-nostdinc++ -I %{include-dir} -I %{libcxx-dir}/test/support'
+))
+config.substitutions.append(('%{link_flags}',
+    '-nostdlib++ -L %{lib-dir} -lc++'
+))
+config.substitutions.append(('%{exec}',
+    '%{executor} --execdir %T -- '
+))
+
+config.stdlib = 'apple-libc++'
+config.using_system_stdlib = True
+
+libcxx.test.config.configure(
+    libcxx.test.params.DEFAULT_PARAMETERS,
+    libcxx.test.features.DEFAULT_FEATURES,
+    config,
+    lit_config
+)

libcxx/test/configs/apple-libc++-system.cfg.in

@@ -10,6 +10,9 @@
 
 // basic_filebuf<charT,traits>* open(const char* s, ios_base::openmode mode);
 
+// In C++23 and later, this test requires support for P2467R1 in the dylib (a3f17ba3febbd546f2342ffc780ac93b694fdc8d)
+// XFAIL: (!c++03 && !c++11 && !c++14 && !c++17 && !c++20) && using-built-library-before-llvm-18
+
 // XFAIL: LIBCXX-AIX-FIXME
 
 #include <fstream>

libcxx/test/std/input.output/file.streams/fstreams/filebuf.members/open_pointer.pass.cpp

@@ -10,6 +10,9 @@
 
 // basic_streambuf<charT, traits>* setbuf(char_type* s, streamsize n) override;
 
+// In C++23 and later, this test requires support for P2467R1 in the dylib (a3f17ba3febbd546f2342ffc780ac93b694fdc8d)
+// XFAIL: (!c++03 && !c++11 && !c++14 && !c++17 && !c++20) && using-built-library-before-llvm-18
+
 #include <fstream>
 #include <cstddef>
 #include <cassert>

libcxx/test/std/input.output/file.streams/fstreams/filebuf.virtuals/setbuf.pass.cpp

@@ -13,6 +13,9 @@
 
 // explicit basic_fstream(const char* s, ios_base::openmode mode = ios_base::in | ios_base::out);
 
+// In C++23 and later, this test requires support for P2467R1 in the dylib (a3f17ba3febbd546f2342ffc780ac93b694fdc8d)
+// XFAIL: (!c++03 && !c++11 && !c++14 && !c++17 && !c++20) && using-built-library-before-llvm-18
+
 // XFAIL: LIBCXX-AIX-FIXME
 
 #include <fstream>

libcxx/test/std/input.output/file.streams/fstreams/fstream.cons/pointer.pass.cpp

@@ -13,6 +13,9 @@
 
 // void open(const char* s, ios_base::openmode mode = ios_base::in|ios_base::out);
 
+// In C++23 and later, this test requires support for P2467R1 in the dylib (a3f17ba3febbd546f2342ffc780ac93b694fdc8d)
+// XFAIL: (!c++03 && !c++11 && !c++14 && !c++17 && !c++20) && using-built-library-before-llvm-18
+
 // XFAIL: LIBCXX-AIX-FIXME
 
 #include <fstream>

libcxx/test/std/input.output/file.streams/fstreams/fstream.members/open_pointer.pass.cpp

@@ -13,6 +13,9 @@
 
 // explicit basic_ofstream(const char* s, ios_base::openmode mode = ios_base::out);
 
+// In C++23 and later, this test requires support for P2467R1 in the dylib (a3f17ba3febbd546f2342ffc780ac93b694fdc8d)
+// XFAIL: (!c++03 && !c++11 && !c++14 && !c++17 && !c++20) && using-built-library-before-llvm-18
+
 // XFAIL: LIBCXX-AIX-FIXME
 
 #include <fstream>

libcxx/test/std/input.output/file.streams/fstreams/ofstream.cons/pointer.pass.cpp

@@ -13,6 +13,9 @@
 
 // void open(const char* s, ios_base::openmode mode = ios_base::out);
 
+// In C++23 and later, this test requires support for P2467R1 in the dylib (a3f17ba3febbd546f2342ffc780ac93b694fdc8d)
+// XFAIL: (!c++03 && !c++11 && !c++14 && !c++17 && !c++20) && using-built-library-before-llvm-18
+
 // XFAIL: LIBCXX-AIX-FIXME
 
 #include <fstream>

libcxx/test/std/input.output/file.streams/fstreams/ofstream.members/open_pointer.pass.cpp

@@ -9,6 +9,9 @@
 // UNSUPPORTED: 32-bit-pointer
 // REQUIRES: large_tests
 
+// This bug was fixed in the dylib by 53aed4759b33e33614e0f4e321bc1ef764b6d5b6.
+// XFAIL: using-built-library-before-llvm-17
+
 // Android devices frequently don't have enough memory to run this test. Rather
 // than throw std::bad_alloc, exhausting memory triggers the OOM Killer.
 // UNSUPPORTED: LIBCXX-ANDROID-FIXME