[libc++][CI] Upgrades clang-format version used.

[Mordante]

Summary

The version used is now determined by Buildkite instead of using the
hard-coded version in the Docker image.

[Mordante]

Reenable the entire CI.

libcxx/utils/ci/buildkite-pipeline.yml

 #
 
 # The Linux CI runners use the nightly ToT build provided by the Docker image.
 # (Note the image isn't updated daily.) The LLVM_HEAD_VERSION contains that
 # version number. The Linux CI runners for GCC use the latest stable version.
 # Theses numbers are available in all runners, making it easier to update the
 # version number.
 env:
+    LLVM_STABLE_VERSION: "15"
     LLVM_HEAD_VERSION: "16"
     GCC_STABLE_VERSION: "12"
 steps:
   #
   # Light pre-commit tests for things like formatting or when people forget
   # to update generated files.
   #
   - label: "Format"
     command: "libcxx/utils/ci/run-buildbot check-format"
     artifact_paths:
       - "**/clang-format.patch"
+    env:
+        GIT_CLANG_FORMAT: "/usr/bin/git-clang-format-${LLVM_STABLE_VERSION}"
     agents:
       queue: "libcxx-builders"
       os: "linux"
     retry:
       automatic:
         - exit_status: -1  # Agent was lost
           limit: 2
     soft_fail:

libcxx/utils/ci/run-buildbot

 if [ ! -z "${CXX}" ]; then ${CXX} --version; fi
 
 case "${BUILDER}" in
 check-format)
     clean
     echo "+++ Checking formatting"
     # We need to set --extensions so that clang-format checks extensionless files.
     mkdir -p ${BUILD_DIR}
-    git-clang-format \
-        --binary /usr/bin/clang-format --diff \
+    if [ -z "${GIT_CLANG_FORMAT}" ]; then
+       GIT_CLANG_FORMAT=git-clang-format
+    fi
+    ${GIT_CLANG_FORMAT} \
+        --diff \
         --extensions ',h,hh,hpp,hxx,c,cc,cxx,cpp' HEAD~1 \
         -- \
             libcxx/{benchmarks,include,src,test} \
             libcxxabi/{fuzz,include,src,test} \
         | tee ${BUILD_DIR}/clang-format.patch
     # Check if the diff is empty, fail otherwise.
     ! grep -q '^--- a' ${BUILD_DIR}/clang-format.patch
 ;;