ci: Use APT_LLVM_V in msan task

Also, use update-alternatives to avoid having to manually specify
clang-${APT_LLVM_V} or llvm-symbolizer-${APT_LLVM_V} everywhere.

ci/test/00_setup_env_native_msan.sh

@@ -7,13 +7,14 @@
 export LC_ALL=C.UTF-8
 
 export CI_IMAGE_NAME_TAG="mirror.gcr.io/ubuntu:24.04"
+export APT_LLVM_V="20"
 LIBCXX_DIR="/msan/cxx_build/"
 export MSAN_FLAGS="-fsanitize=memory -fsanitize-memory-track-origins=2 -fno-omit-frame-pointer -g -O1 -fno-optimize-sibling-calls"
 LIBCXX_FLAGS="-nostdinc++ -nostdlib++ -isystem ${LIBCXX_DIR}include/c++/v1 -L${LIBCXX_DIR}lib -Wl,-rpath,${LIBCXX_DIR}lib -lc++ -lc++abi -lpthread -Wno-unused-command-line-argument"
 export MSAN_AND_LIBCXX_FLAGS="${MSAN_FLAGS} ${LIBCXX_FLAGS}"
 
 export CONTAINER_NAME="ci_native_msan"
-export PACKAGES="ninja-build"
+export PACKAGES="clang-${APT_LLVM_V} llvm-${APT_LLVM_V} llvm-${APT_LLVM_V}-dev libclang-${APT_LLVM_V}-dev libclang-rt-${APT_LLVM_V}-dev ninja-build"
 export DEP_OPTS="DEBUG=1 NO_QT=1 CC=clang CXX=clang++ CFLAGS='${MSAN_FLAGS}' CXXFLAGS='${MSAN_AND_LIBCXX_FLAGS}'"
 export GOAL="install"
 # Setting CMAKE_{C,CXX}_FLAGS_DEBUG flags to an empty string ensures that the flags set in MSAN_FLAGS remain unaltered.