calls: Remove FIXME for cxx17_empty_base_field_p

This predicate is now used by aarch64 targets.

2020-04-29  Richard Sandiford  <richard.sandiford@arm.com>

gcc/
	* calls.h (cxx17_empty_base_field_p): Turn into a function declaration.
	* calls.c (cxx17_empty_base_field_p): New function.  Check
	DECL_ARTIFICIAL and RECORD_OR_UNION_TYPE_P in addition to the
	previous checks.