ci: use libc++ for Clang 5 C++17 coverage

Author: rwgk

.github/workflows/ci.yml

@@ -273,8 +273,11 @@ jobs:
       fail-fast: false
       matrix:
         include:
+          # Clang 5 cannot compile libstdc++ 10's C++17 std::variant due to
+          # LLVM PR31852/PR33222. Its matching libc++ 5 is compatible.
           - clang: 5
             std: 17
+            cxx_flags: "-stdlib=libc++"
           - clang: 11
             std: 20
           - clang: 16