[Ada] Illegal use of current instance in attribute reference

[Arnaud Charlet <charlet at adacore dot com>]

The current instance of a type in an aspect specification is an object of
the type. If the type is scalar, it cannot be the prefix of an attribute
reference such as 'First,  whose prefix must an array object (even though it
can be a scalar type in other contexts).

Compiling foo.adb must yield:
foo.adb:3:29:
  prefix of "First" attribute cannot be the current instance of a scalar type
foo.adb:6:29:
  prefix of "First" attribute cannot be the current instance of a scalar type

---
procedure Foo is
    type T_Data_Sending_Frequency is new Natural
      with Default_Value => T_Data_Sending_Frequency'First;

    type Infrequent is new Natural
      with Default_Value => Infrequent'First + 3;

begin
    null;
end;

Tested on x86_64-pc-linux-gnu, committed on trunk

2016-04-20  Ed Schonberg  <schonberg@adacore.com>

	* sem_attr.adb (Check_Type): Reject an attribute reference in
	an aspect expression, when the prefix of the reference is the
	current instance of the type to which the aspect applies.

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