[ELF] Rework debug line parsing to use llvm::Error and callbacks (LLD-side)

Reviewed by: ruiu, grimar, espindola

Differential Revision: https://reviews.llvm.org/D44562

Summary:
r331971 changes the debug line parser interface to report LLVM errors in an
interface that different executables can use, rather than always being printed
directly as warnings to stderr. This change allows LLD to make use of the new
interface and call its own warning methods to report problems.

llvm-svn: 331972