[Ada] Crash on compilation unit function that builds in place

[Ada] Crash on compilation unit function that builds in place

This patch fixes a crash on a function that builds its limited result in
place. Previously this was handled properly only if the function was a
child unit.

2018-12-11  Ed Schonberg  <schonberg@adacore.com>

gcc/ada/

	* sem_ch3.adb (Build_Itype_Reference): Handle properly an itype
	reference created for a function that is a compilation unit, for
	example if the function builds in place an object of a limited
	type.

gcc/testsuite/

	* gnat.dg/bip_cu.adb, gnat.dg/bip_cu_constructor.adb,
	gnat.dg/bip_cu_constructor.ads, gnat.dg/bip_cu_t.adb,
	gnat.dg/bip_cu_t.ads: New testcase.

From-SVN: r266999