[Ada] fix handling of overriding operations

[Arnaud Charlet <charlet at adacore dot com>]

Tested on i686-linux, committed on trunk.

The homonym chain of an overriding operation may contain inherited
operations created by a previous implementation of an interface. When
checking whether an overriding indicator is consistent with previous
operations of the type, ignore those inherited operations and examine
directly the ancestors in progenitor interfaces.
See gnat.dg/overriding_ops.adb

Also, the cross-reference information for a callable entity includes an
encoding for its formal parameters. This encoding was not being generated
for task and protected entries.

2006-10-31  Hristian Kirtchev  <kirtchev@adacore.com>
	    Ed Schonberg  <schonberg@adacore.com>

	* sem_ch9.adb (Analyze_Protected_Definition): Remove call to
	Check_Overriding_Indicator.
	(Analyze_Task_Definition): Ditto.
	(Analyze_Protected_Type, Analyze_Task_Type): Code cleanup.
	(Check_Overriding_Indicator): To find overridden interface operation,
	examine only homonyms that have an explicit subprogram declaration, not
	inherited operations created by an unrelated type derivation.
	(Check_Overriding_Indicator): When checking for the presence of "null"
	in a procedure, ensure that the queried node is a procedure
	specification.
	(Matches_Prefixed_View_Profile): Add mechanism to retrieve the parameter
	type when the formal is an access to variable.
	(Analyze_Protected_Type): Add check for Preelaborable_Initialization
	(Analyze_Task_Type): Same addition
	(Analyze_Entry_Declaration): Call Generate_Reference_To_Formals, to
	provide navigation capabilities for entries.

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