[Darwin] Stop linking libclang_rt.eprintf.a

Summary:
The eprintf library was added before the general OS X builtins library existed as a place to store one builtin function. Since we have for several years had an actual mandated builtin library for OS X > 10.5, we should just merge eprintf into the main library.

This change will resolve PR28855.

As a follow up I'll also patch compiler-rt to not generate the eprintf library anymore.

Reviewers: ddunbar, bob.wilson

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D23531

llvm-svn: 278988