[llvm] Be explicit about what libstdc++ C++11 ABI to use (PR #154447)

[via llvm-commits]

serge-sans-paille wrote:

It may be worth mentioning https://gcc.gnu.org/onlinedocs/libstdc++/manual/using_dual_abi.html in the commit message. I'm fine with the change and conservatively setting `-D_GLIBCXX_USE_CXX11_ABI=0` looks ok.

I compiled `opt` statically with the two options and I indeed see `AllowFiles[abi:cxx11]` vs `AllowFiles` and other symbols with different suffix, I can confirm the patch works as expected.

https://github.com/llvm/llvm-project/pull/154447