[Ada] Spurious error in dispatching call with class-wide precondition

[Pierre-Marie de Rodat]

This patch fixes a spurious visibility error on a dispatching call to
a subprogram with a classwide precondition, when the call qppears in
the same declarative part as the subprogram declaration itself.

Tested on x86_64-pc-linux-gnu, committed on trunk

2019-08-20  Ed Schonberg  <schonberg@adacore.com>

gcc/ada/

	* exp_disp.adb (Build_Class_Wide_Check, Replace_Formals): When a
	dispatching call tp a subprogram with a class-wide precondition
	occurrs in the same declarative part as the ancestor subprogram
	being called, the`expression for the precondition has not been
	analyzed yet. Such a call may appear, e.g. in an expression
	function. In that case, the replacement of formals by actuals in
	the call cannot use the formal entities of the subprogram being
	called, and the occurrence of the formals in the expression must
	be located by name (Chars fields) as would be done at a later
	freeze point, when the expression is resolved in the context of
	the subprogram itself.

gcc/testsuite/

	* gnat.dg/tagged5.adb, gnat.dg/tagged5.ads: New testcase.
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