[libc++][CI] Updates Docker image.

- Updates the image to use Ubuntu Jammy.
- Installs GCC-12 as preparation to migrate to that GCC version.

NOTE: This is a re-application of f2f0dba, which was reverted
in 2b5e3ef due to an issue with the CI nodes. The CI nodes have
since then been updated and this appears to be fine.

Differential Revision: https://reviews.llvm.org/D126666

Author: mordante

libcxx/utils/ci/Dockerfile

@@ -28,7 +28,7 @@
 #   $ docker push ldionne/libcxx-builder
 #
 
-FROM ubuntu:focal
+FROM ubuntu:jammy
 
 # Make sure apt-get doesn't try to prompt for stuff like our time zone, etc.
 ENV DEBIAN_FRONTEND=noninteractive
@@ -55,11 +55,18 @@ RUN printf "fr_CA ISO-8859-1\ncs_CZ ISO-8859-2" >> /usr/local/share/i1en/SUPPORT
 RUN locale-gen
 
 # Install Clang <latest>, <latest-1> and ToT, which are the ones we support.
-# We also install <latest-2> to allow smooth CI transitions around release points.
+# We also install <latest-2> because we need to support the "latest-1" of the
+# current LLVM release branch, which is effectively the <latest-2> of the
+# tip-of-trunk LLVM. For example, after branching LLVM 14 but before branching
+# LLVM 15, we still need to have Clang 12 in this Docker image because the LLVM
+# 14 release branch CI uses it. The tip-of-trunk CI will never use Clang 12,
+# though.
 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
+# TODO Use the apt.llvm.org version after branching to LLVM 15
+RUN apt-get update && apt-get install -y clang-$(($LLVM_LATEST_VERSION - 2))
+#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
@@ -79,9 +86,9 @@ RUN ln -s /usr/bin/git-clang-format-$LLVM_LATEST_VERSION /usr/bin/git-clang-form
 RUN apt-get install -y clang-tidy-$LLVM_LATEST_VERSION
 RUN ln -s /usr/bin/clang-tidy-$LLVM_LATEST_VERSION /usr/bin/clang-tidy && [ -e $(readlink /usr/bin/clang-tidy) ]
 
-# Install the most recent GCC
-ENV GCC_LATEST_VERSION=11
-RUN add-apt-repository ppa:ubuntu-toolchain-r/test
+# Install the most recent GCC, like clang install the previous version as a transition.
+ENV GCC_LATEST_VERSION=12
+RUN apt-get update && apt install -y gcc-$((GCC_LATEST_VERSION - 1)) g++-$((GCC_LATEST_VERSION - 1))
 RUN apt-get update && apt install -y gcc-$GCC_LATEST_VERSION g++-$GCC_LATEST_VERSION
 
 # Install a recent CMake