[Ada] Temporary tweak new expansion of contracts

[Ada] Temporary tweak new expansion of contracts

In the case of a function, the new expansion of contracts makes use of an
extended return statement to store the result of the function in the return
object while the post-conditions are evaluated.

Unfortunately GNAT does not elide the copy of the return object for extended
return statements for the time being, so this scheme incurs an extra copy of
the return value on the primary or secondary stack, as well as an additional
pair of calls to Adjust/Finalize when the return type needs finalization.

This temporarily changes the expansion to use a block statement containing a
renaming, which does not incur the extra copy provided that it is manually
adjusted to be recognized by the existing "tail call" optimization present
in the Expand_Simple_Function_Return routine.

gcc/ada/

	* contracts.adb (uild_Subprogram_Contract_Wrapper): Remove useless
	local variable. In the case of a function, replace the extended
	return statement by a block statement declaring a renaming of the
	call to the local subprogram after removing side effects manually.
	(Expand_Subprogram_Contract): Adjust description accordingly.
	* exp_ch6.adb (Expand_Ctrl_Function_Call): Rewrite obsolete
	comment and do not apply the transformation twice.
	* sem_attr.adb (Analyze_Attribute_Old_Result): Now expect a block
	statement instead of an extended return statement.