[DebugInfo] Force users of DWARFDebugAbbrev to call parse before iterating

In an attempt to make it easier to catch errors when parsing the
debug_abbrev section, we should force users to call `parse` before
calling `begin`. In a follow-up change, I will change the return type of
`parse` from `void` to `Error`.

I also explored using the fallible_iterator pattern instead of forcing
users to parse everything up front. I think it would be a useful and
interesting pattern to implement, but it would require more extensive
changes to both DWARFDebugAbbrev and its users. Because my top priority
is improving the safety around parsing debug_abbrev, I'm opting to
preserve existing behavior until I or somebody else has time to refactor
to be able to implement a fallible_iterator.

Differential Revision: https://reviews.llvm.org/D154655