ada: Crash on assignment of task allocator with expanded name

ada: Crash on assignment of task allocator with expanded name

The compiler crashes on an assignment statement of the form
"X.Y := new T;", where X.Y is an expanded name (i.e. not a record
component or similar) and T is a type containing tasks.

gcc/ada/ChangeLog:

	* exp_util.adb (Build_Task_Image_Decls):
	Deal properly with the case of an expanded name.
	Minor cleanup: use a case statement instead of if/elsif chain.