[Ada] Wrong value after assignment of overlain record objects

[Ada] Wrong value after assignment of overlain record objects

This patch corrects an issue whereby objects of a record type with a
representation clause which are overlain by address would fail to get
assigned values properly when one or both of said objects were marked
volatile.

2018-07-31  Justin Squirek  <squirek@adacore.com>

gcc/ada/

	* exp_ch5.adb (Make_Field_Assign): Force temporarily generated
	objects for assignment of overlaid user objects to be renamings
	instead of constant declarations.

gcc/testsuite/

	* gnat.dg/addr11.adb: New testcase.

From-SVN: r263098