[libc++] Shows the detailed compiler version info.

[Mordante]

Summary

The libc++ pre-commit CI uses Clang nightly builds. Currently it's not
possible to determine the exact version used since CMake doesn't show
this information by default. Instead use the --version flag to get this
information.

[ldionne]

IMO we should do this in run-buildbot instead. We can add this to the phase at the beginning where we output the --version of various tools (cmake, ninja, etc.). We should be able to use something like if [ ! -z "${CXX}" ]; then ${CXX} --version; fi.

[Mordante]

Use @ldionne's suggested approach.

[ldionne]

libcxx/utils/ci/run-buildbot

150–153

IMO this comment doesn't pull its weight. It's useful to print the compiler version, no justification needed.

libcxx/utils/ci/run-buildbot

 function check-cxx-benchmarks() {
     echo "--- Running the benchmarks"
     ${NINJA} -vC "${BUILD_DIR}" check-cxx-benchmarks
 }
 
 # Print the version of a few tools to aid diagnostics in some cases
 ${CMAKE} --version
 ${NINJA} --version
 
+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 \
         --extensions ',h,hh,hpp,hxx,c,cc,cxx,cpp' HEAD~1 \