[Ada] Tbuild cleanup

[Ada] Tbuild cleanup

gcc/ada/

	* tbuild.adb (Convert_To): Add assert, along with a comment.
	(Make_DT_Access): Remove this function, which is not used.  It
	was incorrect anyway (the call to New_Occurrence_Of should not
	be there).
	(Unchecked_Convert_To): Add assert.  The previous version's test
	for unchecked conversion to the same type was redundant and
	could never be true, because the previous 'if' already checked
	for ANY expression of the same type. Remove that, and replace
	with a test for unchecked conversion to a related type.
	Otherwise, we somethings get things like
	"finalize(some_type!(some_type!(x)))" in the generated code,
	where x is already of type some_type, but we're converting it to
	the private type and then to the full type or vice versa (so the
	types aren't equal, so the previous 'if' doesn't catch it).
	Avoid updating the Parent. This is not necessary; the Parent
	will be updated if/when the node is attached to the tree.
	* tbuild.ads: Fix comments. No need to say "this is safe" when
	we just explained that a few lines earlier.  Remove
	Make_DT_Access.
	* sinfo.ads: Add comments.
	* exp_ch7.adb (Make_Finalize_Address_Stmts): Minor comment fix.
	* gen_il-gen.adb, gen_il-gen.ads, gen_il-gen-gen_nodes.adb,
	gen_il-internals.ads: Implement a feature where you can put:
	Nmake_Assert => "expr" where expr is a boolean expression in a
	call to Create_Concrete_Node_Type. It is added in a pragma
	Assert in the Nmake.Make_... function for that type.