[Ada] gnatbind: Correct assertions in Add_Edge_Kind_Check

[Ada] gnatbind: Correct assertions in Add_Edge_Kind_Check

2020-06-09  Bob Duff  <duff@adacore.com>

gcc/ada/

	* bindo-graphs.ads (Library_Graph_Edge_Kind): Reorder enumerals
	to reflect the order of adding edges. Clarify comments.
	* bindo-graphs.adb (Add_Edge_Kind_Check): Correct the
	assertions.  Reorder the "when"s to match the order of adding
	edges, and therefore the order of enumerals in type
	Library_Graph_Edge_Kind.  Change names to "Old_" and "New_" to
	clarify what's what.  Combine Invocation_Edge into the "<="
	test.  Fix the "raise Program_Error" message, which was
	backwards.