[Ada] Membership test of class-wide interface

[Ada] Membership test of class-wide interface

The compiler rejects the use of a membership test when the left operand
is a class-wide interface type object and the right operand is not a
class-wide type.

2018-05-25  Javier Miranda  <miranda@adacore.com>

gcc/ada/

	* sem_res.adb (Resolve_Membership_Op): Allow the use of the membership
	test when the left operand is a class-wide interface and the right
	operand is not a class-wide type.
	* exp_ch4.adb (Tagged_Membership): Adding support for interface as the
	left operand.

gcc/testsuite/

	* gnat.dg/interface7.adb: New testcase.

From-SVN: r260738