[Ada] Spurious error on prefixed call in an instantiation

[Ada] Spurious error on prefixed call in an instantiation

This patch fixes a spurious error on a prefixed call in an instance, when the
generic parameters include an interface type and an abstract operation of that
type, and the actuals in the instance include an interface type and a
corresponding abstract operation of it, with a different name than the
corresponding generic subprogram parameter. The patch also fixes a similar
error involving class-wide operations and generic private types.

2018-07-17  Ed Schonberg  <schonberg@adacore.com>

gcc/ada/

	* sem_ch4.adb (Try_Object_Operation): Handle properly a prefixed call
	in an instance, when the generic parameters include an interface type
	and a abstract operation of that type, and the actuals in the instance
	include an interface type and a corresponding abstract operation of it,
	with a different name than the corresponding generic subprogram
	parameter.

gcc/testsuite/

	* gnat.dg/generic_call_cw.adb, gnat.dg/generic_call_iface.adb: New
	testcase.

From-SVN: r262803