libtool does not support bitcode files generated by upstream LLVM in the latest
toolchains. When building LLVM with full LTO, we should use llvm-ranlib/llvm-ar.
I am not sure if this is the best way to solve the underlying problem:
being able to do a stage2 bootstrap build of LLVM master with LTO on
Darwin. Unfortunately, CMAKE_AR/CMAKE_RANLIB are automatically populated
if they are not provided. Otherwise we could limit avoiding using
libtool only when the user specified CMAKE_AR/CMAKE_RANLIB manually with LTO.
Any thoughts very appreciated!