Unconditionally pass `-lto_library` to the linker on Darwin

We're only doing it with -flto currently, however it never "hurt"
to pass it, and users that are linking without -flto can get in
trouble if one of the dependency (a static library for instance)
contains bitcode.

Differential Revision: https://reviews.llvm.org/D25932

llvm-svn: 285254