[Ada] Missing accessibility check on access discriminant in extended return

[Ada] Missing accessibility check on access discriminant in extended return

2020-06-04  Justin Squirek  <squirek@adacore.com>

gcc/ada/

	* sem_ch6.adb (Check_Return_Obj_Accessibility): Change to
	Check_Return_Construct_Accessibility to better reflect its
	purpose.  Add loop to properly obtain the object declaration
	from an expanded extended return statement and add calls to get
	the original node for associated values. Also, avoid checks when
	the return statement being examined comes from an internally
	generated function.