[libc++] Install git-clang-format on CI nodes.

[ldionne]

Summary

Two problems fixed:

• an old version of clang-format get installed by default (6.0).
• git-clang-format is not present, only git-clang-format-<version> (e.g. git-clang-format-6.0).

Solution:

• install clang-format-11 with explicit version
• make symlink git-clang-format to the latest version of git-clang-format-<version>

[curdeius]

I have no access to a macOS machine to verify/fix macos-ci-setup script.

[curdeius]

BTW, @ldionne, feel free to commandeer the revision if it makes things easier.

[ldionne]

Actually, this LGTM as-is. On macOS, brew install clang-format also installs git-clang-format.

libcxx/utils/ci/Dockerfile

 FROM ubuntu:bionic
 
 # Make sure apt-get doesn't try to prompt for stuff like our time zone, etc.
 ENV DEBIAN_FRONTEND=noninteractive
 
 RUN apt-get update && apt-get install -y bash curl
 
 # Install various tools used by the build or the test suite
-RUN apt-get update && apt-get install -y ninja-build python3 python3-sphinx python3-distutils git gdb clang-format
+RUN apt-get update && apt-get install -y ninja-build python3 python3-sphinx python3-distutils git gdb clang-format-11
 RUN apt-get update && apt-get install -y libc6-dev-i386 # Required to cross-compile to 32 bits
+# Make a symbolic link to git-clang-format (pointing to git-clang-format-<version>), if it doesn't exist.
+RUN which git-clang-format || $(cd /usr/bin/ && ln -s $(ls git-clang-format-* | sort -rV | head -n 1) git-clang-format)
 
 # Install the most recently released LLVM
 RUN apt-get update && apt-get install -y lsb-release wget software-properties-common
 RUN bash -c "$(wget -O - https://apt.llvm.org/llvm.sh)"
 RUN ln -s $(find /usr/bin -regex '^.+/clang\+\+-[0-9.]+$') /usr/bin/clang++
 RUN ln -s $(find /usr/bin -regex '^.+/clang-[0-9.]+$') /usr/bin/clang
 
 # Install a recent GCC