Remove casts from dwarf2/index-write.c

dwarf2/index-write.c casts pointers to "dwarf2_psymtab *", but as far
as I can tell, it does not actually use any DWARF-specific fields of
the psymtab.  So, this patch changes this code to use partial_symtab
instead.  This removes nearly every cast, leaving just the unavoidable
one from addrmap iteration.

gdb/ChangeLog
2020-02-26  Tom Tromey  <tom@tromey.com>

	* dwarf2/index-write.c (psym_index_map): Change type.
	(add_address_entry_worker, write_one_signatured_type)
	(recursively_count_psymbols, recursively_write_psymbols)
	(class debug_names, psyms_seen_size, write_gdbindex)
	(write_debug_names): Use partial_symtab, not dwarf2_psymtab.