We don’t need to set -stdlib=libstdc++. This only works on Clang so it
is not good to set it globally. In addition, Clang knows to use
libstdc++ on Linux by default if no stdlib is set:
324f918438/lib/Driver/ToolChains/Linux.cpp (L456)
It’s a good policy to just leave off stdlib for now.
Fixes#29877.