Remove unsused variables.
Allow DWARFFormValue::extractValue to be called with a null CU.

Currently FormValues are only used for attributes of DIEs and thus
uers always have a CU lying around when calling into the FormValue
API.
Accelerator tables encode their information using the same Forms
as the attributes, thus it is natural to use DWARFFormValue to
extract/dump them. There is no CU in that case though. Allow the
API to be called with a null CU arguemnt by making the RelocMap
lookup conditional on the CU pointer validity. And document this
new behvior in the header. (Test coverage for this use of the API
comes in the DwarfAccelTable support patch)
[dwarfdump] Add support for dumping accelerator tables.

The class used for the dump only allows to dump for the moment, but
it can (and will) be easily extended to support search also.
Fix emission of Dwarf accelerator table when there are multiple CUs.

The DIE offset in the accel tables is an offset relative to the start
of the debug_info section, but we were encoding the offset to the
start of the containing CU.
Another test for PR19372, showing why we need to keep checking arguments after a pack expansion.