This removes the objfile_psymtabs class in favor of a method on
objfile and on psymtab_storage.

2019-01-16  Tom Tromey  <tom@tromey.com>

	* objfiles.h (struct objfile) <psymtabs>: New method.
	(class objfile_psymtabs): Remove.
	* psymtab.h (class psymtab_storage) <partial_symtab_range>: New
	typedef.
	<range>: New method.
	(require_partial_symbols): Change return type.
	* psymtab.c (require_partial_symbols)
	(psym_expand_symtabs_matching): Update.
	* mdebugread.c (parse_partial_symbols): Update.
	* dbxread.c (dbx_end_psymtab): Update.