Default to GLIBCXX_USE_CXX11_ABI=ON

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 be179d0 related
to #154447.

Author: serge-sans-paille

llvm/cmake/modules/HandleLLVMOptions.cmake

@@ -183,7 +183,7 @@ CHECK_CXX_SOURCE_COMPILES("
 int main() { return 0; }
 " LLVM_USES_LIBSTDCXX)
 
-option(GLIBCXX_USE_CXX11_ABI "Use new libstdc++ CXX11 ABI" OFF)
+option(GLIBCXX_USE_CXX11_ABI "Use new libstdc++ CXX11 ABI" ON)
 
 if (LLVM_USES_LIBSTDCXX)
   if (GLIBCXX_USE_CXX11_ABI)