Change representation of psymbol to flush out accessors

Change representation of psymbol to flush out accessors

This is the psymbol analog to the patch to change the representation
of minimal symbols:

    https://sourceware.org/ml/gdb-patches/2013-10/msg00524.html

It has the same rationale: namely, that we're going to change the code
to apply psymbol offsets at runtime.  This will be done by adding an
argument to the SYMBOL_VALUE_ADDRESS macro -- but since we can't
convert all the symbol types at once, we need a new approach.

Because gdb now is in C++, this patch changes partial_symbol to
inherit from general_symbol_info, rather than renaming the field.
This simplifies code in some places.

Also, as noted before, these macros implement a kind of "phony
polymorphism" that is not actually useful in practice; so this patch
removes the macros in favor of simply referring directly to members.
In a few cases -- obj_section in this patch and the symbol address in
the future -- methods will be used instead.

Note that this removes the blanket memset from add_psymbol_to_bcache.
This hasn't really been needed since bcache was modified to allow
holes in objects and since psymtab took advantage of that.  This
deletion was required due to changing partial_symbol to derive from
general_symbol_info.

gdb/ChangeLog
2018-07-26  Tom Tromey  <tom@tromey.com>

	* dwarf-index-write.c (write_psymbols, debug_names::insert)
	(debug_names::write_psymbols): Update.
	* psympriv.h (struct partial_symbol): Derive from
	general_symbol_info.
	<obj_section>: New method.
	(PSYMBOL_DOMAIN, PSYMBOL_CLASS): Remove.n
	* psymtab.c (find_pc_sect_psymtab_closer, find_pc_sect_psymtab)
	(find_pc_sect_psymbol, fixup_psymbol_section)
	(match_partial_symbol, lookup_partial_symbol, relocate_psymtabs)
	(print_partial_symbols, recursively_search_psymtabs)
	(compare_psymbols, psymbol_hash, psymbol_compare)
	(add_psymbol_to_bcache, maintenance_check_psymtabs)
	(psymbol_name_matches, psym_fill_psymbol_map): Update.