[Ada] Fix wrong call to protected function returning VFA type

[Eric Botcazou]

This is a small regression present on the mainline and 7 branch: the call to a 
protected function returning a composite type with Volatile_Full_Access aspect 
or pragma yields a segfault at run time.

Tested on x86-64/Linux, applied on the mainline and 7 branch.


2018-03-10  Eric Botcazou  <ebotcazou@adacore.com>

	* gcc-interface/trans.c (node_has_volatile_full_access) <N_Identifier>:
	Consider only entities for objects.


2018-03-10  Eric Botcazou  <ebotcazou@adacore.com>

	* gnat.dg/prot3.adb: New test.
	* gnat.dg/prot3_pkg.ad[sb]: New helper.

-- 
Eric Botcazou
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