[DWARF] Add printout for op-index

This is a preparatory patch for extending DWARFDebugLine to properly
parse line number programs with maximum_operations_per_instruction > 1
for VLIW targets.

Add some scaffolding for handling op-index in line number programs, and
add printouts for that in the table. As this affects a lot of tests,
this is done in a separate commit to get a cleaner review for the actual
op-index implementation.

Verbose printouts are not present in many tests, and adding op-index to
those will require a bit more code changes, so that is done in the
actual implementation patch.

Reviewed By: StephenTozer

Differential Revision: https://reviews.llvm.org/D152535