Remove last objfile partial_symtab references from psymtab.c

Remove last objfile partial_symtab references from psymtab.c

This removes the last references to the partial_symtab via the objfile
from psymtab.c.  require_partial_symbols is now a method on
psymbol_functions, and maintenance_print_psymbols is changed to use
dynamic_cast to verify that it is examining partial symbols.

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

	* psymtab.c (psymbol_functions::require_partial_symbols): Rename.
	(psymbol_functions::find_pc_sect_psymtab): Rename.
	(psymbol_functions::find_pc_sect_compunit_symtab)
	(maintenance_print_psymbols, maintenance_check_psymtabs): Update.
	* psympriv.h (struct psymbol_functions) <require_partial_symbols>:
	Declare new method.
	<get_partial_symtabs, find_pc_sect_psymtab>: Likewise.