Add dwarf2_per_bfd::start_reading

Add dwarf2_per_bfd::start_reading

The cooked index "start_reading" method can only be called after the
dwarf2_per_bfd "index_table" member is set.  This patch refactors this
code a little to centralize this constraint, adding a new
dwarf2_per_bfd::start_reading method and another (virtual) method to
dwarf_scanner_base.

This removes some casts, but also is also useful to support another
series I'm working on where the .gdb_index is rewritten.

Approved-By: Simon Marchi <simon.marchi@efficios.com>