Add some new DW_IDX_* constants

I've reimplemented the .debug_names code in GDB -- it was quite far
from being correct, and the new implementation is much closer to what
is specified by DWARF.

However, the new writer in GDB needs to emit some symbol properties,
so that the reader can be fully functional.  This patch adds a few new
DW_IDX_* constants, and tries to document the existing extensions as
well.  (My patch series add more documentation of these to the GDB
manual as well.)

include/ChangeLog
2023-12-10  Tom Tromey  <tom@tromey.com>

	* dwarf2.def (DW_IDX_GNU_internal, DW_IDX_GNU_external): Comment.
	(DW_IDX_GNU_main, DW_IDX_GNU_language, DW_IDX_GNU_linkage_name):
	New constants.