Fix .debug_names regression with new indexer

At AdaCore, we run the internal gdb test suite in several modes,
including one using the .debug_names index.  This caught a regression
caused by the new DWARF indexer.

First, the psymtabs-based .debug_names generator was completely wrong.
However, to avoid making the rewrite series even bigger (fixing the
writer will also require rewriting the .debug_names reader), it
attempted to preserve the weirdness.

However, this was not done properly.  For example the old writer did
this:

-      case STRUCT_DOMAIN:
-	return DW_TAG_structure_type;

The new code, instead, simply preserves the actual DWARF tag -- but
this makes future lookups fail, because the .debug_names reader only
looks for DW_TAG_structure_type.

This patch attempts to revert to the old behavior in the writer.