Don't use objfile::intern in DWO code

Don't use objfile::intern in DWO code

The DWO code in the DWARF reader currently uses objfile::intern.  This
accesses a shared data structure and so would be racy when used from
multiple threads.  I don't believe this can happen right now, but
background reading could provoke this, and in any case it's better to
avoid this, just to be sure.

This patch changes this code to just use a std::string.  A new type is
introduced to do hash table lookups, to avoid unnecessary copies.