Introduce objfile::require_partial_symbols

Introduce objfile::require_partial_symbols

This adds a new method, objfile::require_partial_symbols.  This reuses
most of the code from the old function in psymtab.c.  That function is
now made static, and simplified.

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

	* symfile.c (read_symbols): Use objfile method.
	* symfile-debug.c (objfile::require_partial_symbols): New method.
	* psymtab.h (require_partial_symbols): Don't declare.
	* psymtab.c (require_partial_symbols): Use objfile method.  Now
	static.
	(psymbol_functions::map_symtabs_matching_filename, OBJFILE)
	(psymbol_functions::lookup_symbol)
	(psymbol_functions::lookup_global_symbol_language)
	(psymbol_functions::find_last_source_symtab)
	(psymbol_functions::forget_cached_source_info)
	(psymbol_functions::print_stats)
	(psymbol_functions::expand_symtabs_for_function)
	(psymbol_functions::expand_all_symtabs)
	(psymbol_functions::expand_symtabs_with_fullname)
	(psymbol_functions::map_symbol_filenames)
	(psymbol_functions::map_matching_symbols)
	(psymbol_functions::expand_symtabs_matching)
	(psymbol_functions::find_compunit_symtab_by_address)
	(maintenance_print_psymbols, maintenance_info_psymtabs)
	(maintenance_check_psymtabs): Update.
	* objfiles.h (struct objfile) <require_partial_symbols>: Declare
	new method.