This new version has tapi support, which is needed to build the new
stubs based libSystem, etc. and Big Sur support.

You can verify the provenance of these yourself by checking Hydra here:
https://hydra.nixos.org/build/128192471

Adapted from main expression for clang 7.

Build the llvm support libraries (libcxx, libcxxabi) from scratch
without using the existing llvm libraries. This is the same spirit and
similar implementation as the "useLLVM" bootstrap in llvm package
sets. Critically it avoids having libcxxabi provided by the cc-wrapper
when building libcxx, which otherwise results in two libcxxabi
instances.

$ otool -L /nix/store/vd4vvgs9xngqbjzpg3qc41wl6jh42s9i-libc++-7.1.0/lib/libc++.dylib
/nix/store/vd4vvgs9xngqbjzpg3qc41wl6jh42s9i-libc++-7.1.0/lib/libc++.dylib:
        /nix/store/vd4vvgs9xngqbjzpg3qc41wl6jh42s9i-libc++-7.1.0/lib/libc++.1.0.dylib (compatibility version 1.0.0, current version 1.0.0)
        /nix/store/gmpwk5fyp3iasppqrrdpswxvid6kcp8r-libc++abi-7.1.0/lib/libc++abi.dylib (compatibility version 1.0.0, current version 1.0.0)
        /nix/store/3hn7azynqgp2pm5gpdg45gpq0ia72skg-libc++abi-7.1.0/lib/libc++abi.dylib (compatibility version 1.0.0, current version 1.0.0)
        /nix/store/1nq94scbxs6bk7pimqhvz76q6cfmbv97-Libsystem-osx-10.12.6/lib/libSystem.B.dylib (compatibility version 1.0.0, current version 1226.10.1)

Additionally move some utilities (clang, binutils, coreutils, gnugrep)
to the stage layers so they can be replaced before the final
stdenv. This should cause most of stage4 to be built from the
toolchain assembled as of stage3 instead of the bootstrap toolchain.

Exposed by the tbd stubs which contains fat libraries. The previously
used proxy libraries were x86_64 only.

Used during bootstrap to check that re-exported libraries are not
dangling.