[Ada] Volatility, validity checks, and System.Aux_DEC

[Ada] Volatility, validity checks, and System.Aux_DEC

This patch updates validity checks to prevent the validation of an
by-reference formal parameter because the parameter is not being read in
the process.

2018-12-11  Hristian Kirtchev  <kirtchev@adacore.com>

gcc/ada/

	* checks.adb: Add with and use clauses for Sem_Mech.
	(Ensure_Valid): Update the "annoying special case" to include
	entry and function calls. Use Get_Called_Entity to obtain the
	entry or subprogram being invoked, rather than retrieving it
	manually. Parameters passed by reference do not need a validity
	check.

gcc/testsuite/

	* gnat.dg/valid4.adb, gnat.dg/valid4_pkg.adb,
	gnat.dg/valid4_pkg.ads: New testcase.

From-SVN: r267012