CI: Add Ubuntu Clang task

This feels more like a band-aid solution - it still requires very manual
intervention to get the next builds, but at least this way we don't rely
on whatever Clang/libc++ happens to ship with FreeBSD for the only
libc++ testing. It seems like sanitizer builds don't use libc++, but I'd
rather a dedicated Linux/Clang/libc++ task at least.

.cirrus.yml

@@ -268,6 +268,18 @@ ubuntu24_task:
     << : *RESOURCES_TEMPLATE
   << : *CI_TEMPLATE
 
+# Same as above, but using Clang and libc++
+ubuntu24_clang_libcpp_task:
+  container:
+    # Ubuntu 24.04 EOL: Jun 2029
+    dockerfile: ci/ubuntu-24.04/Dockerfile
+    << : *RESOURCES_TEMPLATE
+  << : *CI_TEMPLATE
+  env:
+    CC: clang-18
+    CXX: clang++-18
+    CXXFLAGS: -stdlib=libc++
+
 ubuntu22_task:
   container:
     # Ubuntu 22.04 EOL: June 2027

ci/ubuntu-24.04/Dockerfile

@@ -36,6 +36,8 @@ RUN apt-get update && apt-get -y install \
     unzip \
     wget \
     zlib1g-dev \
+    libc++-dev \
+    libc++abi-dev \
   && apt autoclean \
   && rm -rf /var/lib/apt/lists/*