ada: Fix crash on expression function returning tagged type in nested package

ada: Fix crash on expression function returning tagged type in nested package

This happens when the expression is a reference to a formal parameter of
the function, or a conditional expression with such a reference as one of
its dependent expressions, because the RM 6.5(8/5) subclause prescribes a
tag reassignment in this case, which requires freezing the tagged type in
the GNAT freezing model, although the language says there is no freezing.

In other words, it's another occurrence of the discrepancy between this
model tailored to Ada 95 and the freezing rules introduced in Ada 2012,
that is papered over by Should_Freeze_Type and the associated processing.

gcc/ada/

	* exp_util.ads (Is_Conversion_Or_Reference_To_Formal): New
	function declaration.
	* exp_util.adb (Is_Conversion_Or_Reference_To_Formal): New
	function body.
	* exp_ch6.adb (Expand_Simple_Function_Return): Call the predicate
	Is_Conversion_Or_Reference_To_Formal in order to decide whether a
	tag check or reassignment is needed.
	* freeze.adb (Should_Freeze_Type): Move declaration and body to
	the appropriate places. Also return True for tagged results
	subject to the expansion done in Expand_Simple_Function_Return
	that is guarded by the predicate
	Is_Conversion_Or_Reference_To_Formal.