[libcxx] [llvm] Add libc++ github actions workflow to replace buildkite (PR #71836)

[via llvm-commits]

================
@@ -101,291 +101,13 @@ environment_definitions:
 
 
 steps:
-  #
-  # General testing with the default configuration, under all the supported
-  # Standard modes, with Clang and GCC. This catches most issues upfront.
-  # The goal of this step is to catch most issues while being very fast.
-  #
-- label: GCC ${GCC_STABLE_VERSION} / C++latest
-  command: libcxx/utils/ci/run-buildbot generic-gcc
-  env:
-    <<: *common_env
-    CC: gcc-${GCC_STABLE_VERSION}
-    CXX: g++-${GCC_STABLE_VERSION}
-  <<: *common
-  <<: *linux_agent
-  
-
-
-- label: C++26
-  command: libcxx/utils/ci/run-buildbot generic-cxx26
-  env:
-    <<: *common_env
-    <<: *absolute_path_clang
-  <<: *linux_agent
-  <<: *common
-
-- label: Modular build
-  command: libcxx/utils/ci/run-buildbot generic-modules
-  env:
-    <<: *common_env
-  <<: *linux_agent
-  <<: *common
-
-- label: C++11
-  command: libcxx/utils/ci/run-buildbot generic-cxx11
-  env:
-    <<: *common_env
-  <<: *linux_agent
-  <<: *common
-
-- label: C++03
-  command: libcxx/utils/ci/run-buildbot generic-cxx03
-  env:
-    <<: *common_env
-  <<: *linux_agent
-  <<: *common
-
-  #
-  # All other supported configurations of libc++.
-  #
-- wait
-
-- label: C++23
-  command: libcxx/utils/ci/run-buildbot generic-cxx23
-  env:
-    <<: *common_env
-    <<: *absolute_path_clang
-    ENABLE_STD_MODULES: 'On'
----------------
EricWF wrote:

Done, except for std::modules, which previously hijacked the C++23 and C++26 build, making them not follow the "test as our users do" principle for basic use of C++23 and C++26. As a replacement, I added a stage 3 job that runs with std::modules in C++26

https://github.com/llvm/llvm-project/pull/71836