ada: Sort cross-reference table using heap and not stack

ada: Sort cross-reference table using heap and not stack

Cross-references are used by GNATprove for code that is not in SPARK. They are
sorted using an auxiliary array. This array should be allocated on the heap and
not on stack, because it can be arbitrarily large, especially for
auto-generated code.

gcc/ada/ChangeLog:

	* lib-xref.adb (Output_References): Put local array object on the heap.