ada: Remove exception handler in Check_Vanishing_Fields

ada: Remove exception handler in Check_Vanishing_Fields

Check_Vanishing_Fields calls Same_Node_To_Fetch_From, which was calling
Node_To_Fetch_From, which will raise an exception (as it should) on
..._Type_Only fields if called early (when the Base_Type,
Implementation_Base_Type, or Root_Type has not yet been set).
Other exceptions can also be raised when Check_Vanishing_Fields
is called early.

An exception handler was used to ignore any such exceptions. That was
correct, but it is annoying in gdb with "catch exception". (Note that
efficiency doesn't matter much, because Check_Vanishing_Fields is not
done in production mode.)

We now call Node_To_Fetch_From_If_Set, remove other potential raises,
and remove the now-unnecessary exception handler.

Note that other calls to Node_To_Fetch_From will still raise an
exception (as they should) for such "early" calls.

gcc/ada/ChangeLog:

	* atree.adb (Same_Node_To_Fetch_From):
	Use Node_To_Fetch_From_If_Set, and remove handler.
	* einfo-utils.adb (Root_Type_If_Set):
	Return Empty if Base_Type_If_Set returns Empty.
	Return Empty if we find Etype (T) = Empty.
	(Underlying_Type): Use "Has_Non_Limited_View..." instead
	of "Present (Non_Limited_View...)", because the latter raises
	an exception when given the wrong Ekind.