[libc++] Add a job testing on GCC 11

[ldionne]

Summary

I'm adding the job as a soft-fail for now, but once all the tests have
been fixed to work on it, we'll switch over from GCC 10 to GCC 11 and
remove the soft-fail.

[ldionne]

Poke CI

[ldionne]

Fix buildkite-pipeline.yml

[ldionne]

Rebase onto main, which was broken.

[ldionne]

ASAN and TSAN are currently known to fail, and @zoecarver is working on those failures. Shipping this since it doesn't cause additional issues beyond the existing ones.

libcxx/utils/ci/buildkite-pipeline.yml

artifact_paths:
       - "**/test-results.xml"
     agents:
       queue: "libcxx-builders"
     retry:
       automatic:
         - exit_status: -1  # Agent was lost
           limit: 2
 
+  - label: "GCC-next/C++20"
+    command: "libcxx/utils/ci/run-buildbot generic-gcc-next"
+    artifact_paths:
+      - "**/test-results.xml"
+    agents:
+      queue: "libcxx-builders"
+    retry:
+      automatic:
+        - exit_status: -1  # Agent was lost
+          limit: 2
+    soft_fail:
+      - exit_status: 1
+
   #
   # All other supported configurations of libc++.
   #
   - wait
 
   - label: "-fno-exceptions"
     command: "libcxx/utils/ci/run-buildbot generic-noexceptions"
     artifact_paths:

libcxx/utils/ci/run-buildbot

 ;;
 generic-gcc)
     export CC=gcc
     export CXX=g++
     clean
     generate-cmake
     check-cxx-cxxabi
 ;;
+generic-gcc-next)
+    export CC=gcc-11
+    export CXX=g++-11
+    clean
+    generate-cmake
+    check-cxx-cxxabi
+;;
 generic-asan)
     export CC=clang
     export CXX=clang++
     clean
     generate-cmake -C "${MONOREPO_ROOT}/libcxx/cmake/caches/Generic-asan.cmake"
     check-cxx-cxxabi
 ;;
 generic-msan)