Fix method naming bug in new DWARF indexer

Pedro pointed out that gdb-add-index is much slower with the new DWARF
indexer.  He also noticed that, in some cases, the generated
.gdb_index would have the wrong fully-qualified name for a method.

I tracked this down to a bug in the indexer.  If a type could have
methods but was marked as a declaration, the indexer was ignoring it.
However, this meant that the internal map to find the qualified name
was not updated for this container.