[Ada] Equality for nonabstract type derived from interface treated as abstract

[Ada] Equality for nonabstract type derived from interface treated as abstract

The compiler was creating an abstract function for the equality
operation of a (nonlimited) interface type, and that could result in
errors on generic instantiations that are passed nonabstract types
derived from the interface type along with the derived type's inherited
equality operation (complaining about an abstract subprogram being
passed to a nonabstract formal). The "=" operation of an interface is
supposed to be nonabstract (a direct consequence of the rule in RM
4.5.2(6-7)), so we now create an expression function rather than an
abstract function. The function returns False, but the result is
unimportant since a function of an abstract type can never actually be
invoked (its arguments must generally be class-wide, since there can be
no objects of the type, and calling it will dispatch).

2019-08-14  Gary Dismukes  <dismukes@adacore.com>

gcc/ada/

	* exp_ch3.adb (Predef_Spec_Or_Body): For an equality operation
	of an interface type, create an expression function (that
	returns False) rather than declaring an abstract function.
	* freeze.adb (Check_Inherited_Conditions): Set Needs_Wrapper to
	False unconditionally at the start of the loop creating wrappers
	for inherited operations.

gcc/testsuite/

	* gnat.dg/equal11.adb, gnat.dg/equal11_interface.ads,
	gnat.dg/equal11_record.adb, gnat.dg/equal11_record.ads: New
	testcase.

From-SVN: r274464