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

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.

Reviewed By: #libc, ldionne

Differential Revision: https://reviews.llvm.org/D133122

libcxx/utils/ci/run-buildbot

@@ -148,6 +148,8 @@ function check-cxx-benchmarks() {
 ${CMAKE} --version
 ${NINJA} --version
 
+if [ ! -z "${CXX}" ]; then ${CXX} --version; fi
+
 case "${BUILDER}" in
 check-format)
     clean