
Because many of our bots actually don't run a listdc++ compatible with _GLIBCXX_USE_CXX11_ABI=0. See https://gcc.gnu.org/onlinedocs/libstdc++/manual/using_dual_abi.html for details. This is a follow-up to be179d069664ce03c485e49fa1f6e2ca3d6286fa related to #154447.
See docs/CMake.html for instructions on how to build LLVM with CMake.