Zig now depends on LLVM 5.0.0.

For the latest version that supports LLVM 4.0.1, use
2a49c876be76dc98996a3251310728ad32b22363.

Unfortunately we had to embed LLD into Zig due to some
MACH-O related LLD bugs. One of them is already upstream
and another is awaiting feedback on the llvm-dev mailing
list.

You can use cmake option -DZIG_FORCE_EXTERNAL_LLD=ON to
still use external LLD if you want to live with the MACH-O
bugs or if your system LLD is patched.

Closes #273