ada: Handle call to C++ constructor placed in return statement

Add missing support for C++ constructor call placed in the return
statement.

gcc/ada/ChangeLog:

	* sem_ch3.adb (Analyze_Object_Declaration): Report an error if the
	return statement of a function with foreign convention has a call
	to a C++ constructor.
	* exp_ch3.adb (Expand_N_Object_Declaration): Handle initialization
	expression calling a C++ constructor in an extended return statement
	of a subprogram with Ada convention.