dwarf2_per_bfd::index_addrmap is only used by the .gdb_index reader,
so this field can be moved to mapped_gdb_index instead. Then,
cooked_index_functions::find_per_cu can be removed in favor of a
method on the index object.
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=31821
Approved-By: Simon Marchi <simon.marchi@efficios.com>