Add llvm-tli-checker

A new tool that compares TargetLibraryInfo's opinion of the availability
of library function calls against the functions actually exported by a
specified set of libraries. Can be helpful in verifying the correctness
of TLI for a given target, and avoid mishaps such as had to be addressed
in D107509 and 94b4598d.

The tool currently supports ELF object files only, although it's unlikely
to be hard to add support for other formats.

Differential Revision: https://reviews.llvm.org/D111358