ada: Change syntax for destructors extension

ada: Change syntax for destructors extension

This patch replaces aspect-based version of the destructors extension
with a new version that uses the "direct attribute definition" syntax
that was recently introduced by the constructors extension.

gcc/ada/ChangeLog:

	* snames.ads-tmpl: Make "Destructor" an attribute name.
	* snames.adb-tmpl: Allow direct attribute definition for Destructor.
	* gen_il-fields.ads (Destructor): New field.
	(Is_Destructor): Remove.
	* gen_il-gen-gen_entities.adb: (Destructor): New field.
	(Is_Destructor): Remove.
	* einfo.ads (Destructor): Document new field.
	(Is_Destructor): Remove documentation.
	* aspects.ads (Aspect_Destructor): Remove.
	* exp_attr.adb (Expand_N_Attribute_Reference): Adapt after aspect
	removal.
	* exp_ch7.adb (Build_Finalize_Statements): Adapt to new destructor
	representation.
	* freeze.adb (Freeze_Entity): Remove obsolete check.
	* sem_attr.adb (Analyze_Attribute, Eval_Attribute): Adapt to new
	attribute.
	* sem_ch13.adb (Analyze_Aspect_Specifications,
	Check_Aspect_At_End_Of_Declarations): Adapt after aspect removal.
	* sem_ch6.adb (Analyze_Direct_Attribute_Definition): Add handling
	of Destructor attribute.
	(Can_Be_Destructor_Of): New function.
	* doc/gnat_rm/gnat_language_extensions.rst: Adapt documentation to
	new syntax.
	* gnat_rm.texi: Regenerate.
	* gnat_ugn.texi: Regenerate.