[libc++] Remove the CI job for Apple/System/Noexceptions

[ldionne]

Summary

When we recently started using DYLD_LIBRARY_PATH to run the test suite
on the Apple/System configuration of the library, the -fno-exceptions
variant started failing.

It started failing because under that configuration, libc++abi.dylib
doesn't provide support for exceptions. For example, it doesn't provide
some symbols such as ___gxx_personality_v0. Now, the problem is that
when the test suite is run with DYLD_LIBRARY_PATH, /usr/lib/libobjc.dylib
uses the just-built libc++abi.dylib, which doesn't support exceptions,
and we end up with an unresolved reference to ___gxx_personality_v0.

Previously, using -Wl,-rpath,path/to/lib, we would be loading both
/usr/lib/libc++abi.dylib and <just-built>/lib/libc++abi.dylib.
/usr/lib/libobjc.dylib would use the system libc++abi.dylib, which
contains support for exceptions, and the tests would be using the
just-built one, which doesn't.

Disentangling that led me to believe that we shouldn't try to test this
configuration where libc++/libc++abi are built as system libraries, but
where they don't support exceptions, since that just doesn't make any
sense. Doing so is like trying to build libc++/libc++abi and test it as
a system library after performing an ABI break -- of course nothing is
going to work.

For that reason, I am removing this configuration. Note that we could
still test the library on macOS without exceptions if we wanted, only
we wouldn't be building it as a system library. This patch doesn't add
that because we already have a -fno-exceptions CI job on Linux.

libcxx/utils/ci/buildkite-pipeline.yml

agents:
       queue: "libcxx-builders"
       os: "macos"
     retry:
       automatic:
         - exit_status: -1  # Agent was lost
           limit: 2
     timeout_in_minutes: 120
 
-  # TODO: Re-enable this once issues with the Apple-specific testing config have been resolved
-  # - label: "Apple system -fno-exceptions"
-  #   command: "libcxx/utils/ci/run-buildbot x86_64-apple-system-noexceptions"
-  #   artifact_paths:
-  #     - "**/test-results.xml"
-  #   agents:
-  #     queue: "libcxx-builders"
-  #     os: "macos"
-  #   retry:
-  #     automatic:
-  #       - exit_status: -1  # Agent was lost
-  #         limit: 2
-  #   timeout_in_minutes: 120
-
   # Test back-deployment to older Apple platforms
   - label: "Apple back-deployment macosx10.9"
     command: "libcxx/utils/ci/run-buildbot x86_64-apple-system-backdeployment-10.9"
     artifact_paths:
       - "**/test-results.xml"
     agents:
       queue: "libcxx-builders"
       os: "macos10.15" # TODO: For now, we're running the back-deployment tests for 10.9 on 10.15, because we don't have proper 10.9 machines

libcxx/utils/ci/run-buildbot

generic-no-unicode)
     check-runtimes
 ;;
 x86_64-apple-system)
     clean
     generate-cmake -C "${MONOREPO_ROOT}/libcxx/cmake/caches/Apple.cmake" \
                    -DLIBCXX_TEST_CONFIG="apple-libc++-shared.cfg.in"
     check-runtimes
 ;;
-x86_64-apple-system-noexceptions)
-    clean
-    generate-cmake -C "${MONOREPO_ROOT}/libcxx/cmake/caches/Apple.cmake" \
-                   -DLIBCXX_ENABLE_EXCEPTIONS=OFF \
-                   -DLIBCXXABI_ENABLE_EXCEPTIONS=OFF \
-                   -DLIBCXX_TEST_CONFIG="apple-libc++-shared.cfg.in"
-    check-runtimes
-;;
 x86_64-apple-system-backdeployment-*)
     clean
 
     if [[ "${OSX_ROOTS}" == "" ]]; then
         echo "--- Downloading previous macOS dylibs"
         PREVIOUS_DYLIBS_URL="https://dl.dropboxusercontent.com/s/liu4fmc53qzlfly/libcxx-roots.tar.gz"
         OSX_ROOTS="${BUILD_DIR}/macos-roots"
         mkdir -p "${OSX_ROOTS}"