[Ada] Pragma Unreferenced triggers undefined reference

[Ada] Pragma Unreferenced triggers undefined reference

This patch corrects the generation of protected body declarations so
that instances of pragma Unreferenced applied to formals don't falsly
trigger undefined references.

2019-07-11  Justin Squirek  <squirek@adacore.com>

gcc/ada/

	* exp_ch9.adb (Build_Private_Protected_Declaration): Add
	exception for the moving of pragmas to internally generated
	specs for pragma Unreferenced.

gcc/testsuite/

	* gnat.dg/unreferenced2.adb: New testcase.

From-SVN: r273392