[runtimes] Officially deprecate the legacy testing configuration system

Add a warning and tweak the release note to explain that the deprecation
targets libc++, libc++abi and libuwnind as well.

Also, as a fly-by, ensure that our CI runs the legacy testing configuration
for libc++, libc++abi and libunwind. This doesn't matter too much since
it's deprecated, but we might as well test it properly.

Differential Revision: https://reviews.llvm.org/D126478

libcxx/docs/ReleaseNotes.rst

@@ -148,7 +148,8 @@ Build System Changes
   its own headers.
 
 - The legacy testing configuration is now deprecated and will be removed in the next release. For
-  most users, this should not have any impact. However, if you are testing libc++ in a configuration
-  or on a platform that used to be supported by the legacy testing configuration and isn't supported
-  by one of the configurations in ``libcxx/test/configs``, please reach out to the libc++ developers
-  to get your configuration supported officially.
+  most users, this should not have any impact. However, if you are testing libc++, libc++abi or
+  libunwind in a configuration or on a platform that used to be supported by the legacy testing
+  configuration and isn't supported by one of the configurations in ``libcxx/test/configs``,
+  ``libcxxabi/test/configs`` or ``libunwind/test/configs``, please move to one of those
+  configurations or define your own.

libcxx/test/configs/legacy.cfg.in

@@ -62,3 +62,6 @@ import libcxx.test.config
 configuration = libcxx.test.config.Configuration(lit_config, config)
 configuration.configure()
 configuration.print_config_info()
+
+lit_config.warning("This is a legacy testing configuration which will be removed in LLVM 16. "
+                   "Please use one of the configurations in libcxx/test/configs or define your own.")

libcxx/utils/ci/run-buildbot

@@ -462,7 +462,9 @@ bootstrapping-build)
 ;;
 legacy-test-config)
     clean
-    generate-cmake -DLIBCXX_TEST_CONFIG="legacy.cfg.in"
+    generate-cmake -DLIBCXX_TEST_CONFIG="legacy.cfg.in" \
+                   -DLIBCXXABI_TEST_CONFIG="${MONOREPO_ROOT}/libcxxabi/test/lit.site.cfg.in" \
+                   -DLIBUNWIND_TEST_CONFIG="${MONOREPO_ROOT}/libunwind/test/lit.site.cfg.in"
     check-runtimes
 ;;
 legacy-project-build)

libcxxabi/test/lit.site.cfg.in

@@ -60,3 +60,6 @@ import libcxxabi.test.config
 configuration = libcxxabi.test.config.Configuration(lit_config, config)
 configuration.configure()
 configuration.print_config_info()
+
+lit_config.warning("This is a legacy testing configuration which will be removed in LLVM 16. "
+                   "Please use one of the configurations in libcxxabi/test/configs or define your own.")

libunwind/test/lit.site.cfg.in

@@ -56,3 +56,6 @@ import libunwind.test.config
 configuration = libunwind.test.config.Configuration(lit_config, config)
 configuration.configure()
 configuration.print_config_info()
+
+lit_config.warning("This is a legacy testing configuration which will be removed in LLVM 16. "
+                   "Please use one of the configurations in libunwind/test/configs or define your own.")