ada: Preserve AST structure when copying tree with discrete types

ada: Preserve AST structure when copying tree with discrete types

When copying AST we created an orphaned copy of a scalar range. This was
confusing the compile-time evaluator, because we couldn't determine location of
a scalar range within an IF statement by looking at its parenthood chain.

gcc/ada/ChangeLog:

	* sem_util.adb (Update_Semantic_Fields): Preserve tree structure when
	copying scalar range of a discrete type.