Revert "[clang] Don't spuriously pass -stdlib=libc++ to CC1 on Darwin"

This broke the instrprof-darwin-exports.c test on mac, see e.g.
https://green.lab.llvm.org/green/job/clang-stage1-RA/32351/

> Previously, we would be passing down -stdlib=libc++ from the Driver
> to CC1 whenever the default standard library on the platform was libc++,
> even if -stdlib= had not been passed to the Driver. This meant that we
> would pass -stdlib=libc++ in nonsensical circumstances, such as when
> compiling C code.
>
> This logic had been added in b534ce46 to make sure that header
> search paths were set up properly. However, since libc++ is now the
> default Standard Library on Darwin, passing this explicitly is not
> required anymore. Indeed, if no -stdlib= is specified, CC1 will end
> up using libc++ if it queries which standard library to use, without
> having to be told.
>
> Not passing -stdlib= at all to CC1 on Darwin should become possible
> once CC1 stops relying on it to set up framework search paths.
>
> Furthermore, this commit also removes a diagnostic checking whether the
> deployment target is too old to support libc++. Nowadays, all supported
> deployment targets use libc++ and compiling with libstdc++ is not
> supported anymore. The Driver was the wrong place to issue this
> diagnostic since it doesn't know whether libc++ will actually be linked
> against (e.g. C vs C++), which would lead to spurious diagnostics.
> Given that these targets are not supported anymore, we simply drop
> the diagnostic instead of trying to refactor it into CC1.
>
> rdar://103198514
>
> Differential Revision: https://reviews.llvm.org/D139938

This reverts commit 6540f32d.