Remove reference to dwarf2_per_cu_data::dwarf2_per_objfile in dw2_do_instantiate_symtab
This patch begins by removing the per_cu->dwarf2_per_objfile reference
in dw2_do_instantiate_symtab, instead accepting a dwarf2_per_objfile
object as a parameter. It then fixes the fallouts. In this context,
the dwarf2_per_objfile is generally derived from an objfile passed to a
quick_symbol_functions callback.