DebugInfo/DWARF: Provide some (pretty half-hearted) error handling access when parsing units

This isn't the most robust error handling API, but does allow clients to
opt-in to getting Errors they can handle. I suspect the long-term
solution would be to move away from the lazy unit parsing and have an
explicit step that parses the unit and then allows access to the other
APIs that require a parsed unit.

llvm-dwarfdump could be expanded to use this (or newer/better API) to
demonstrate the benefit of it - but for now lld will use this in a
follow-up cl which ensures lld can exit non-zero on errors like this (&
provide more descriptive diagnostics including which object file the
error came from).

(error access to later errors when parsing nested DIEs would be good too
- but, again, exposing that without it being a hassle for every consumer
may be tricky)

llvm-svn: 368377

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