Make some DWARF complaints clearer

Make some DWARF complaints clearer

I noticed that the complaint in partial_die_parent_scope was not using
dwarf_tag_name, so I changed that.  Then I noticed that dwarf_tag_name
does not show the numeric value for an unrecognized tag, so I changed
that function and all the related functions to do so.

gdb/ChangeLog
2019-05-29  Tom Tromey  <tromey@adacore.com>

	* dwarf2read.c (partial_die_parent_scope): Call dwarf_tag_name.
	(dwarf_unknown): New function.
	(dwarf_tag_name, dwarf_attr_name, dwarf_form_name)
	(dwarf_type_encoding_name): Use dwarf_unknown.