[llvm-objdump] Print method name from debug info in disassembly output.

Summary:
GNU objdump prints the method name in disassembly output, and upon further investigation this seems to come from debug info, not the symbol table.

Some additional refactoring is necessary to make this work even when the line number is 0/the filename is unknown. The added test case includes a note for this scenario.

See http://llvm.org/PR41341 for more info.

Reviewers: dblaikie, MaskRay, jhenderson

Reviewed By: MaskRay

Subscribers: ormris, jvesely, aprantl, kerbowa, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D74507