[libc++] Bump the version of Clang we use in the CI

[ldionne]

Note that I will need to wait for the llvm.sh script to be updated to support LLVM 15 before I can actually ship this.

[Mordante]

Nice, I was actually thinking about whether we should do something like this. Especially since with the LLVM 13 release our ToT was out of date for a long time and the switch to the new ToT was not smooth.

[ldionne]

Indeed, I'm able to build the image locally now. I'll ship this -- the CI failure has to be unrelated, since this change isn't tested through CI anyways (the runners use the checked-in version of the image, not the one in this review).

libcxx/utils/ci/Dockerfile

RUN apt-get update && apt-get install -y language-pack-en language-pack-fr \
                                          language-pack-ru language-pack-zh-hans
 # These two are not enabled by default so generate them
 RUN printf "fr_CA ISO-8859-1\ncs_CZ ISO-8859-2" >> /etc/locale.gen
 RUN mkdir /usr/local/share/i1en/
 RUN printf "fr_CA ISO-8859-1\ncs_CZ ISO-8859-2" >> /usr/local/share/i1en/SUPPORTED
 RUN locale-gen
 
 # Install Clang <latest>, <latest-1> and ToT, which are the ones we support.
-ENV LLVM_LATEST_VERSION=13
+# We also install <latest-2> to allow smooth CI transitions around release points.
+ENV LLVM_LATEST_VERSION=14
 RUN apt-get update && apt-get install -y lsb-release wget software-properties-common
 RUN wget https://apt.llvm.org/llvm.sh -O /tmp/llvm.sh
+RUN bash /tmp/llvm.sh $(($LLVM_LATEST_VERSION - 2)) # for CI transitions
 RUN bash /tmp/llvm.sh $(($LLVM_LATEST_VERSION - 1)) # previous release
 RUN bash /tmp/llvm.sh $LLVM_LATEST_VERSION          # latest release
 RUN bash /tmp/llvm.sh $(($LLVM_LATEST_VERSION + 1)) # current ToT
 
 # Make the ToT Clang the "default" compiler on the system
 RUN ln -fs /usr/bin/clang++-$(($LLVM_LATEST_VERSION + 1)) /usr/bin/c++ && [ -e $(readlink /usr/bin/c++) ]
 RUN ln -fs /usr/bin/clang-$(($LLVM_LATEST_VERSION + 1)) /usr/bin/cc && [ -e $(readlink /usr/bin/cc) ]