[test][llvm-dwarfdump][NFC] Improve test readability

This patch adds and improves comments in the debug_line_invalid.test and
its associated input file so that it is easier to follow. It uses '##'
to make comments stand out from lit and FileCheck commands.

It also reflows some commands so that the lines are not so long and are
easier to read and fixes some copy/paste errors.

Reviewed by: JDevlieghere

Differential Revision: https://reviews.llvm.org/D71752