Docker.system-clang: Update the system-clang testcase to Ubuntu 24.04

Update to using clang-18 from the host system; version 16 or
newer is required for the implicitly selected config files
(which are used since 866d47cced);
that version changed whether the target triples are normalized
before locating config files.

This allows building runtimes with the current versions of LLVM
19.x, rather than needing to select an older version.

Dockerfile.system-clang

@@ -1,4 +1,4 @@
-FROM ubuntu:22.04
+FROM ubuntu:24.04
 
 RUN apt-get update -qq && \
     DEBIAN_FRONTEND="noninteractive" apt-get install -qqy --no-install-recommends \
@@ -10,7 +10,7 @@ RUN apt-get update -qq && \
 
 # Separate step for system-clang; the RUN line above is similar to other dockerfiles.
 RUN apt-get update -qq && \
-    apt-get install -qqy clang-15 lld-15 llvm-15 && \
+    apt-get install -qqy clang-18 lld-18 llvm-18 && \
     apt-get clean -y && \
     rm -rf /var/lib/apt/lists/*
 
@@ -33,8 +33,7 @@ ARG CFGUARD_ARGS=--enable-cfguard
 
 COPY build-all.sh build-llvm.sh install-wrappers.sh build-mingw-w64.sh build-mingw-w64-tools.sh build-compiler-rt.sh build-libcxx.sh build-mingw-w64-libraries.sh build-openmp.sh ./
 COPY wrappers/*.sh wrappers/*.c wrappers/*.h wrappers/*.cfg ./wrappers/
-# Libcxx version 17 requires Clang >= 15, while Libcxx 18 requires Clang >= 16.
-RUN LLVM_VERSION=llvmorg-17.0.6 ./build-all.sh $TOOLCHAIN_PREFIX --host-clang=clang-15 && \
+RUN ./build-all.sh $TOOLCHAIN_PREFIX --host-clang=clang-18 && \
     rm -rf /build/*
 
 ENV PATH=$TOOLCHAIN_PREFIX/bin:$PATH