ci: add a Docker container for clang support of C++20 modules

.gitlab/ci/docker/clang_cxx_modules/Dockerfile

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+FROM fedora:37
+MAINTAINER Ben Boeckel <ben.boeckel@kitware.com>
+
+# Install build dependencies for packages.
+COPY install_deps.sh /root/install_deps.sh
+RUN sh /root/install_deps.sh
+
+COPY install_llvm.sh /root/install_llvm.sh
+RUN sh /root/install_llvm.sh
+
+# Install build dependencies for CMake's CI.
+COPY install_cmake_deps.sh /root/install_cmake_deps.sh
+RUN sh /root/install_cmake_deps.sh

.gitlab/ci/docker/clang_cxx_modules/install_cmake_deps.sh

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+#!/bin/sh
+
+set -e
+
+dnf install -y --setopt=install_weak_deps=False \
+    file git-core
+dnf clean all

.gitlab/ci/docker/clang_cxx_modules/install_deps.sh

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+#!/bin/sh
+
+set -e
+
+dnf install -y --setopt=install_weak_deps=False \
+    gcc-c++ cmake ninja-build
+dnf clean all

.gitlab/ci/docker/clang_cxx_modules/install_llvm.sh

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+#!/bin/sh
+
+set -e
+
+readonly revision="p1689r5-cmake-ci-20221201" # ae3b5489585b60a2b7f090ebb9e1e8729b811253
+readonly tarball="https://github.com/mathstuf/llvm-project/archive/$revision.tar.gz"
+
+readonly workdir="$HOME/llvm"
+readonly srcdir="$workdir/llvm"
+readonly builddir="$workdir/build"
+
+mkdir -p "$workdir"
+cd "$workdir"
+curl -L "$tarball" > "llvm-$revision.tar.gz"
+tar xf "llvm-$revision.tar.gz"
+mv "llvm-project-$revision" "$srcdir"
+mkdir -p "$builddir"
+cd "$builddir"
+cmake -GNinja \
+    -DCMAKE_BUILD_TYPE=Release \
+    -DBUILD_SHARED_LIBS=ON \
+    -DLLVM_ENABLE_BINDINGS=OFF \
+    -DLLVM_INCLUDE_BENCHMARKS=OFF \
+    -DLLVM_INCLUDE_DOCS=OFF \
+    -DLLVM_INCLUDE_EXAMPLES=OFF \
+    -DLLVM_INCLUDE_RUNTIMES=OFF \
+    -DLLVM_INCLUDE_TESTS=OFF \
+    -DLLVM_INCLUDE_UTILS=OFF \
+    -DLLVM_TARGETS_TO_BUILD=X86 \
+    -DLLVM_TOOL_CLANG_BUILD=ON \
+    -DLLVM_USE_SYMLINKS=ON \
+    "-DLLVM_EXTERNAL_CLANG_SOURCE_DIR=$srcdir/clang" \
+    -DLLVM_PARALLEL_LINK_JOBS=1 \
+    -DCLANG_BUILD_TOOLS=ON \
+    "-DCMAKE_INSTALL_PREFIX=/opt/llvm-p1689" \
+    "$srcdir/llvm"
+ninja
+ninja install/strip
+rm -rf "$workdir"