[Ada] Spurious "Duplicated symbol" error with discriminated tasks

[Ada] Spurious "Duplicated symbol" error with discriminated tasks

This patch fixes a spurious error in a program that contains a
discriminated task type and several of its subtype in the same
declarative part, when the corresponding discriminant constraints are
expressions.

2018-08-21  Ed Schonberg  <schonberg@adacore.com>

gcc/ada/

	* sem_util.ads, sem_util.adb (New_External_Entity): Type of
	Suffix_Index must be Int, not Nat, so that a negative value can
	be used to generate a unique name for an external object, as
	specified in Tbuild.New_External_Name.
	(Scope_Within): Handle private type whose completion is a
	synchronized type (For unnesting).
	* itypes.ads, itypes.adb (Create_Itype): Ditto
	* sem_ch3.adb (Constrain_Corresponding_Record): Generate a
	unique name for the created subtype, because there may be
	several discriminated tasks present in the same scope, and each
	needs its distinct corresponding record subtype.

gcc/testsuite/

	* gnat.dg/task1.adb, gnat.dg/task1.ads, gnat.dg/task1_pkg.adb,
	gnat.dg/task1_pkg.ads: New testcase.

From-SVN: r263716