Ada: Fix bogus error for 'Value invoked on function call and -gnatVa

This happens when the function takes an In Out or Out parameter, so only in
Ada 2012 and later.  The mechanism used to implement the validity check for
the call, required by -gnatVa, inserts the copy-out statement incorrectly.

gcc/ada/
	PR ada/126379
	* exp_ch6.adb (Insert_Post_Call_Actions): Also deal with attribute
	references as parent node.

gcc/testsuite/
	* gnat.dg/validity_check3.adb: New test.