Handle member pointers directly in generic_value_print

TYPE_CODE_MEMBERPTR and TYPE_CODE_METHODPTR are only used for C++, so
it seems to me that the generic value-printing code ought to handle
these cases -- that way, printing these objects will work even when
the current language is not C++.  This patch implements this idea.

gdb/ChangeLog
2020-09-15  Tom Tromey  <tom@tromey.com>

	* rust-lang.c (rust_value_print_inner): Remove TYPE_CODE_MEMBERPTR
	and TYPE_CODE_METHODPTR cases.
	* c-valprint.c (c_value_print_memberptr): Move to valprint.c.
	(c_value_print_inner): Update.
	* valprint.c (generic_value_print_memberptr): New function, from
	c_value_print_memberptr.
	(generic_value_print): Use it.  Call cplus_print_method_ptr.