Remove sym_fns::sym_read_psymbols

Remove sym_fns::sym_read_psymbols

Partial symbols are read via the sym_fns::sym_read_psymbols function
pointer.  In order to separate the partial symbols from the objfile,
this must instead be done via a virtual method on
quick_symbol_functions.  This patch implements this change.

gdb/ChangeLog
2021-03-20  Tom Tromey  <tom@tromey.com>

	* xcoffread.c (xcoff_sym_fns): Update.
	* symfile.h (struct sym_fns) <sym_read_psymbols>: Remove.
	* symfile-debug.c (objfile::has_partial_symbols): Use
	can_lazily_read_symbols.
	(debug_sym_read_psymbols): Remove.
	(debug_sym_fns, install_symfile_debug_logging): Update.
	* quick-symbol.h (struct quick_symbol_functions)
	<can_lazily_read_symbols, read_partial_symbols>: New methods.
	* psymtab.c (require_partial_symbols): Use new 'qf' methods.
	* mipsread.c (ecoff_sym_fns): Update.
	* machoread.c (macho_sym_fns): Update.
	* elfread.c (struct lazy_dwarf_reader): New.
	(elf_symfile_read): Update.
	(read_psyms): Now a method of lazy_dwarf_reader.
	(elf_sym_fns): Update.
	(elf_sym_fns_lazy_psyms): Remove.
	* dbxread.c (aout_sym_fns): Update.
	* coffread.c (coff_sym_fns): Update.