[Ada] No_Task_Parts aspect

[Pierre-Marie de Rodat]

The patch implements the No_Task_Parts aspect, which says that a type
and extensions of it cannot contain tasks.

In addition, we optimize away tasking-related code for T'Class when T
has No_Task_Parts. This is mostly just an efficiency improvement, but it
fixes a bug in one obscure case: The expanded code for a "for ... of"
loop contains an object of a class-wide type (e.g.
Reversible_Iterator'Class), so the compiler would normally generate
tasking-related calls, which are blocking, so if the procedure
containing "for ... of" is called from a protected object, and
Detect_Blocking is enabled, Program_Error will be raised. These compiler
changes, along with adding No_Task_Parts to the iterator interfaces,
fixes that bug.

Tested on x86_64-pc-linux-gnu, committed on trunk

gcc/ada/

	* aspects.ads (No_Task_Parts): New aspect.
	* snames.ads-tmpl: Add the aspect name.
	* exp_ch6.adb (Might_Have_Tasks): Return False if this is a
	class-wide type whose specific type has No_Task_Parts.
	* freeze.adb (Check_No_Parts_Violations): This is an adaptation
	of the procedure formerly known as
	Check_No_Controlled_Parts_Violations, which now supports both
	No_Controlled_Parts and No_Task_Parts.  It takes a parameter
	indicating which aspect is being checked.
	(Freeze_Entity): Call Check_No_Parts_Violations for both
	aspects.
	* sem_ch13.adb (Analyze_Aspect_Specifications): The code for
	Aspect_No_Controlled_Parts already works as is with
	Aspect_No_Task_Parts.
	* libgnat/a-iteint.ads: Add No_Task_Parts aspect to the two
	iterator iterfaces.
	* doc/gnat_rm/implementation_defined_aspects.rst: Add
	documentation for the No_Task_Parts aspect.
	* gnat_rm.texi: Regenerate.
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