[Ada] Crash on protected type with self-referential component

[Ada] Crash on protected type with self-referential component

This patch fixes a compiler abort on a declarastion for a protected type
PT when one of its private component is of type access PT.

2019-07-11  Ed Schonberg  <schonberg@adacore.com>

gcc/ada/

	* exp_ch9.adb (Expand_N_Protected_Type_Declaaration): New
	subsidiary routine Replace_Access_Definition, to handle properly
	a protected type PT one of whose private components is of type
	access PT.

gcc/testsuite/

	* gnat.dg/prot8.adb, gnat.dg/prot8.ads: New testcase.

From-SVN: r273399