[Ada] Fix spurious loop warning for function with Out parameter

[Ada] Fix spurious loop warning for function with Out parameter

The compiler gives a spurious warning about a possible infinite while
loop whose condition contains a call to a function that takes an Out or
In/Out parameter and whose actual is a variable that is not modified in
the loop, because it still thinks that functions can only have In
parameters.

2019-07-22  Eric Botcazou  <ebotcazou@adacore.com>

gcc/ada/

	* sem_warn.adb (Find_Var): Bail out for a function call with an
	Out or In/Out parameter.

gcc/testsuite/

	* gnat.dg/warn23.adb: New testcase.

From-SVN: r273673