ada: Incorrect creation of corresponding expression of class-wide contracts

GNAT was incorrectly implementing the Ada rules for resolving calls to
primitive functions within inherited class-wide pre- and postconditions,
as specified in RM22 6.1.1 (relating to AI12-0113).  Only function calls
that involve formals of the associated primitive subprogram should be
treated using the "(notional) formal derived type" rules.  In particular,
calls that are tag-indeterminate (for example, "F(G)") should not be mapped
to call the corresponding primitives of the derived type (they should still
call the primitives of the ancestor type).  The fix for this involves a new
predicate function that recursively traverses calls to determine the calls
that satisfy the criteria for mapping.  These changes also completely remove
the mapping of formals that was done in Contracts.Merge_Class_Conditions
(in Inherit_Condition), since the mapping will be done later anyway by
Build_Class_Wide_Expression, and the earlier mapping interferes with that.

Note: The utility function Sem_Util.Check_Parents is no longer called
after removal of the single call to it from contracts.adb, but it's being
retained (along with the generic subprograms in Atree that it depends on)
for possible use in VAST.

gcc/ada/ChangeLog:

	* contracts.adb (Inherit_Condition): Remove Assoc_List and its uses
	along with function Check_Condition, since mapping of formals will
	effectively be done in Build_Class_Wide_Expression (by Replace_Entity).
	* exp_util.adb (Replace_Entity): Only rewrite entity references in
	function calls that qualify according to the result of calling the
	new function Call_To_Parent_Dispatching_Op_Must_Be_Mapped.
	(Call_To_Parent_Dispatching_Op_Must_Be_Mapped): New function that
	determines whether a function call to a primitive of Par_Subp
	associated tagged type needs to be mapped (according to whether
	it has any actuals that reference controlling formals of the
	primitive).

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