ada: Fix premature finalization of return temporary

ada: Fix premature finalization of return temporary

Various parts of the expander and the code generator must have a consistent
view on which temporaries generated for return statements must be finalized
because they are regular temporaries, and which ones must not be since they
are allocated on the return stack directly.  The Is_Related_To_Func_Return
predicate is used for this purpose and needs to be tested consistently.

gcc/ada/

	* exp_ch6.adb (Expand_Simple_Function_Return): Make sure that a
	captured function call also verifies Is_Related_To_Func_Return.
	Do not generate an actual subtype for special return objects.
	* exp_util.ads (Is_Related_To_Func_Return): Add commentary.