[libc++] Add a Lit configuration for running back-deployment tests

[ldionne]

Summary

This testing configuration links tests against one libc++ shared library,
but runs them against another libc++ shared library. This makes sure that
we can build applications against the libc++ provided in a recent SDK and
back-deploy them to platforms containing older libc++ dylibs.

It also switches the Apple CI script to using that new configuration
instead of the legacy one.

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

+# Testing configuration for back-deployment against older Apple system libc++.
+#
+# Under this configuration, we compile and link all the test suite against the latest libc++,
+# however we run against the libc++ on a different platform. This emulates the workflow of
+# a developer building their application using recent tools but with the goal of deploying
+# on existing devices running an older OS (and hence an older dylib).
+
+import os, site
+site.addsitedir(os.path.join('@LIBCXX_SOURCE_DIR@', 'utils'))
+import libcxx.test.params, libcxx.test.newconfig, libcxx.test.dsl
+
+lit_config.load_config(config, '@CMAKE_CURRENT_BINARY_DIR@/cmake-bridge.cfg')
+
+BACKDEPLOYMENT_PARAMETERS = [
+    libcxx.test.dsl.Parameter(name='cxx_runtime_root', type=str,
+        actions=lambda root: [libcxx.test.dsl.AddSubstitution('%{cxx-runtime-root}', root)],
+        help="""
+        The simulated root of the system (for libc++) when running tests.
+
+        This should be a directory hierarchy under which the libc++ dylib can be found.
+        The dylib in that hierarchy is the one that will be used at runtime when running
+        the tests.
+        """),
+    libcxx.test.dsl.Parameter(name='abi_runtime_root', type=str,
+        actions=lambda root: [libcxx.test.dsl.AddSubstitution('%{abi-runtime-root}', root)],
+        help="""
+        The simulated root of the system (for libc++abi) when running tests.
+
+        This should be a directory hierarchy under which the libc++abi dylib can be found.
+        The dylib in that hierarchy is the one that will be used at runtime when running
+        the tests.
+        """),
+]
+
+config.substitutions.append(('%{flags}',
+    '-isysroot {}'.format('@CMAKE_OSX_SYSROOT@') if '@CMAKE_OSX_SYSROOT@' else ''
+))
+config.substitutions.append(('%{compile_flags}',
+    '-nostdinc++ -isystem %{install}/include/c++/v1 -I %{libcxx}/test/support'
+))
+config.substitutions.append(('%{link_flags}',
+    '-nostdlib++ -L %{install}/lib -lc++'
+))
+config.substitutions.append(('%{exec}',
+    '%{executor} --execdir %T --env DYLD_LIBRARY_PATH="%{cxx-runtime-root}:%{abi-runtime-root}" -- '
+))
+
+libcxx.test.newconfig.configure(
+    libcxx.test.params.DEFAULT_PARAMETERS + BACKDEPLOYMENT_PARAMETERS,
+    libcxx.test.features.DEFAULT_FEATURES,
+    config,
+    lit_config
+)

libcxx/utils/ci/run-buildbot

apple-system-backdeployment-*)
 
     arch="$(uname -m)"
     PARAMS="target_triple=${arch}-apple-macosx${DEPLOYMENT_TARGET}"
     PARAMS+=";cxx_runtime_root=${OSX_ROOTS}/macOS/libc++/${DEPLOYMENT_TARGET}"
     PARAMS+=";abi_runtime_root=${OSX_ROOTS}/macOS/libc++abi/${DEPLOYMENT_TARGET}"
     PARAMS+=";use_system_cxx_lib=True"
 
     generate-cmake -C "${MONOREPO_ROOT}/libcxx/cmake/caches/Apple.cmake" \
+                   -DLIBCXX_TEST_CONFIG="apple-libc++-backdeployment.cfg.in" \
                    -DLIBCXX_TEST_PARAMS="${PARAMS}" \
                    -DLIBCXXABI_TEST_PARAMS="${PARAMS}"
 
     check-runtimes
 ;;
 benchmarks)
     clean
     generate-cmake -DLIBCXX_TEST_CONFIG="llvm-libc++-shared.cfg.in" \