[Ada] Spurious warning about a useless assignment

[Ada] Spurious warning about a useless assignment

This patch removes a spurious warning about a useless assignment, when a
composite object is the target of an assignment and is an actual for an
out parameter in a subsewuent call, and there is an intervening use of
the object as the prefix of a selected component in an intervening
operation.

2019-07-22  Ed Schonberg  <schonberg@adacore.com>

gcc/ada/

	* sem_res.adb (Resolve_Selected_Component): If the prefix has a
	deferred reference, generate the correct reference now, to
	indicate that the previous assignment is used.  This prevents
	spurious warnings on useless assignments when compiling with all
	warnings enabled. when there is a subsequent call in the same
	stqtement list, in which the prefix of the selected component is
	the actual for an out parameter.

gcc/testsuite/

	* gnat.dg/warn22.adb: New testcase.

From-SVN: r273669