[Ada] Proper freezing for dispatching expression functions.

[Ada] Proper freezing for dispatching expression functions.

In the case of an expression function that is a primitive function of a
tagged type, freezing the tagged type needs to freeze the function (and
its return expression). A bug in this area could result in incorrect
behavior both at compile time and at run time. At compile time, freezing
rule violations could go undetected so that an illegal program could be
incorrectly accepted. At run time, a dispatching call to the primitive
function could end up dispatching through a not-yet-initialized slot in
the dispatch table, typically (although not always) resulting in a
segmentation fault.

gcc/ada/

	* freeze.adb (Check_Expression_Function.Find_Constant): Add a
	check that a type that is referenced as the prefix of an
	attribute is fully declared.
	(Freeze_And_Append): Do not freeze the profile when freezing an
	expression function.
	(Freeze_Entity): When a tagged type is frozen, also freeze any
	primitive operations of the type that are expression functions.
	* sem_ch6.adb (Analyze_Subprogram_Body_Helper): Do not prevent
	freezing associated with an expression function body if the
	function is a dispatching op.