ada: Internal compiler error for Sequential Partition_Elaboration_Policy

In some cases, compilation of a function with a limited class-wide result
type could fail with an internal error if a Sequential
Partition_Elaboration_Policy is specified. To prevent this, we want specifying
a Sequential Partition_Elaboration_Policy to have the side effect of
imposing a No_Task_Hierarchy restriction. But doing that in a straightforward
way leads to problems with incorrectly accepting violations of H.6(6). So
a new restriction, No_Task_Hierarchy_Implicit, is introduced.

gcc/ada/

	* libgnat/s-rident.ads: Define a new restriction,
	No_Task_Hierarchy_Implicit. This is like the No_Task_Hierarchy
	restriction, but with the difference that setting this restriction
	does not mean the H.6(6) post-compilation check is satisified.
	* exp_ch6.adb (Add_Task_Actuals_To_Build_In_Place_Call): If it is
	known that the function result cannot have tasks, then pass in a
	null literal for the activation chain actual parameter. This
	avoids generating a reference to an entity that
	Build_Activation_Chain_Entity may have chosen not to generate a
	declaration for.
	* gnatbind.adb (List_Applicable_Restrictions): Do not list the
	No_Task_Hierarchy_Implicit restriction.
	* restrict.adb: Special treatment for the
	No_Task_Hierarchy_Implicit restriction in functions
	Get_Restriction_Id and Restriction_Active. The former is needed to
	disallow the (unlikely) case that a user tries to explicitly
	reference the No_Task_Hierarchy_Implicit restriction.
	* sem_prag.adb (Analyze_Pragma): If a Sequential
	Partition_Elaboration_Policy is specified (and the
	No_Task_Hierarchy restriction is not already enabled), then enable
	the No_Task_Hierarchy_Implicit restriction.

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