[Ada] Missing check on outbound parameter of a non-null access type

[Pierre-Marie de Rodat]

This patch adds code to generate proper post-call checks when an actual
for an in-out or out parameter has a non-null access type. No
constraints are applied to an inbound access parameter, but on exit a
not-null check must be performed if the type of the actual requires it.

Tested on x86_64-pc-linux-gnu, committed on trunk

2019-08-12  Ed Schonberg  <schonberg@adacore.com>

gcc/ada/

	* exp_ch6.adb (Expand_Actuals. Add_Call_By_Copy_Code): Add code
	to generate proper checks when an actual for an in-out or out
	parameter has a non-null access type.  No constraints are
	applied to an inbound access parameter, but on exit a not-null
	check must be performed if the type of the actual requires it.

gcc/testsuite/

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