Add a DWARFContext& member in DWARFUnit.

The DWARFContext will be used to pass global 'context' down, like
pointers to related debug info sections or command line options.
The first use will be for the debug_info dumper to be able to access
other debug info section to dump eg. Location Expression inline
in the debug_info dump.
Add DWARFFormValue::getAsBlock() and add FC_Flag as an acceptable class for an unsigned constant.

To be used in further patches that improve the dumpers.