[libc++] [CI] Install Tip-of-Trunk clang.

[curdeius]

Summary

• Check created symlinks.

[curdeius]

I can make it work without the LLVM_TOT_VERSION by getting $LLVM_VERSION from llvm.sh script and putting it into a temporary file (because I can't store it in a variable during docker build). Would it be better?
Another thing, should we prefer installing a fixed version of clang-format or ToT? The latter has improvements for concepts for example, but may (theoretically) be unstable.

[curdeius]

• Put current LLVM_VERSION in file and use it later.
• Make script completely automatic. Don't rely on LLVM_VERSION variable from llvm.sh.

[ldionne]

This LGTM - did you test it? Build the image and run one of the build bots (./libcxx/utils/ci/run-buildbot whatever). If it passes, LGTM.

[curdeius]

Yes, built and tested. Works fine. Will land it soon.

libcxx/utils/ci/Dockerfile

 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
 RUN apt-get update && apt-get install -y libc6-dev-i386 # Required to cross-compile to 32 bits
 
 # 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
-
-RUN apt-get update && apt-get install -y clang-format-11
-# 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 $(ls git-clang-format-* | sort -rV | head -n 1) git-clang-format)
+RUN wget https://apt.llvm.org/llvm.sh -O /tmp/llvm.sh
+RUN bash /tmp/llvm.sh
+RUN LLVM_VERSION=$(find /usr/bin -regex '^.+/clang-[0-9.]+$') && LLVM_VERSION=${LLVM_VERSION#*clang-} && echo "LLVM_VERSION=$LLVM_VERSION" > /tmp/env.sh
+RUN ln -s $(find /usr/bin -regex '^.+/clang\+\+-[0-9.]+$') /usr/bin/clang++ && [ -e $(readlink /usr/bin/clang++) ]
+RUN ln -s $(find /usr/bin -regex '^.+/clang-[0-9.]+$') /usr/bin/clang && [ -e $(readlink /usr/bin/clang) ]
+
+# Install the not-yet-released LLVM
+RUN . /tmp/env.sh && echo "LLVM_TOT_VERSION=$(($LLVM_VERSION + 1))" >> /tmp/env.sh
+RUN . /tmp/env.sh && bash /tmp/llvm.sh ${LLVM_TOT_VERSION}
+RUN . /tmp/env.sh && ln -s /usr/bin/clang++-${LLVM_TOT_VERSION} /usr/bin/clang++-tot && [ -e $(readlink /usr/bin/clang++-tot) ]
+RUN . /tmp/env.sh && ln -s /usr/bin/clang-${LLVM_TOT_VERSION} /usr/bin/clang-tot && [ -e $(readlink /usr/bin/clang-tot) ]
+
+# Install clang-format
+RUN . /tmp/env.sh && apt-get install -y clang-format-$LLVM_VERSION
+RUN ln -s $(find /usr/bin -regex '^.+/clang-format-[0-9.]+$') /usr/bin/clang-format && [ -e $(readlink /usr/bin/clang-format) ]
+RUN ln -s $(find /usr/bin -regex '^.+/git-clang-format-[0-9.]+$') /usr/bin/git-clang-format && [ -e $(readlink /usr/bin/git-clang-format) ]
 
 # Install a recent GCC
 RUN add-apt-repository ppa:ubuntu-toolchain-r/test
 RUN apt-get update && apt install -y gcc-10 g++-10
-RUN ln -f -s /usr/bin/g++-10 /usr/bin/g++
-RUN ln -f -s /usr/bin/gcc-10 /usr/bin/gcc
+RUN ln -f -s /usr/bin/g++-10 /usr/bin/g++ && [ -e $(readlink /usr/bin/g++) ]
+RUN ln -f -s /usr/bin/gcc-10 /usr/bin/gcc && [ -e $(readlink /usr/bin/gcc) ]
 
 # Install a recent CMake
 RUN wget https://github.com/Kitware/CMake/releases/download/v3.18.2/cmake-3.18.2-Linux-x86_64.sh -O /tmp/install-cmake.sh
 RUN bash /tmp/install-cmake.sh --prefix=/usr --exclude-subdir --skip-license
 RUN rm /tmp/install-cmake.sh
 
 # Change the user to a non-root user, since some of the libc++ tests
 # (e.g. filesystem) require running as non-root. Also setup passwordless sudo.