ada: Fix detection of function calls in object declarations

ada: Fix detection of function calls in object declarations

The current code has relied on Original_Node to detect rewritten function
calls in object declarations but that's not robust enough in the presence
of function calls written in object notation.

gcc/ada/

	* exp_util.ads (Is_Captured_Function_Call): Declare.
	* exp_util.adb (Is_Captured_Function_Call): New predicate.
	* exp_ch3.adb (Expand_N_Object_Declaration): Use it to detect a
	rewritten function call as the initializing expression.
	* exp_ch6.adb (Expand_Simple_Function_Return): Use it to detect a
	rewritten function call as the returned expression.