Print discriminators when printing .debug_line in GNU style.

Summary:
gnu addr2line prints DWARF line table discriminators like so:

<file>:<line> (discriminator <Number>)

This matches that behavior.

Document how and when --output-style=GNU prints discriminators

Add test for new GNU-style discriminator printing.

Reviewers: rupprecht, labath, jhenderson

Subscribers: aprantl, hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D73318