ada: Optimize interface objects initialized with function calls

ada: Optimize interface objects initialized with function calls

This optimizes the implementation of (class-wide) interface objects that are
initialized with function calls, by avoiding an unnecessary copy operation.
This also removes useless access checks generated by the expansion of return
statements involving class-wide types.

gcc/ada/

	* exp_ch3.adb (Expand_N_Object_Declaration): Factor out conditions
	needed for an initializating expression that is a function call to
	be renamable into the Is_Renamable_Function_Call predicate.
	Use it to implement the renaming in the case of class-wide interface
	objects.  Remove an interface conversion on all paths, separate and
	optimize the renaming path in the special expansion for interfaces.
	(Is_Renamable_Function_Call): New predicate.
	(Make_Allocator_For_Return): Put back an interface conversion.
	* exp_ch6.adb (Apply_CW_Accessibility_Check): Remove useless access
	checks on RE_Tag_Ptr.