Avoid .debug_str lookups.

[markj]

Summary

libdwarf caches a string table handle, use that instead. This provides
a noticeable speedup when iterating through DIEs, since we would
otherwise do a linear search of all of the DWARF sections looking for
".debug_str" using strcmp().

[emaste]

Presumably _dwarf_strtab_init is guaranteed to be called?

[markj]

In D20759#449026, @emaste wrote:

Presumably _dwarf_strtab_init is guaranteed to be called?

The only way it won't be called is if an earlier error occurs during initialization. If there is no string table, then the assertion in the removed code would fail.