[Ada] Catch newly illegal case of Unrestricted_Access

[Arnaud Charlet <charlet at adacore dot com>]

It is now illegal to use Unrestricted_Access to directly generate a
thin pointer of an unconstrained array type which references a non-
aliased object. This never worked, and we might as well catch it as
illegal, since it is not hard to do so, as shown in the following
example:

     1. with System; use System;
     2. procedure SliceUA2 is
     3.    type A is access all String;
     4.    for A'Size use Standard'Address_Size;
     5.
     6.    procedure P (Arg : A) is
     7.    begin
     8.       null;
     9.    end P;
    10.
    11.    X : String := "hello world!";
    12.    X2 : aliased String := "hello world!";
    13.
    14.    AV : A := X'Unrestricted_Access;    -- ERROR
                     |
        >>> illegal use of Unrestricted_Access attribute
        >>> attempt to generate thin pointer to unaliased object

    15.
    16. begin
    17.    P (X'Unrestricted_Access);          -- ERROR
              |
        >>> illegal use of Unrestricted_Access attribute
        >>> attempt to generate thin pointer to unaliased object

    18.    P (X(7 .. 12)'Unrestricted_Access); -- ERROR
              |
        >>> illegal use of Unrestricted_Access attribute
        >>> attempt to generate thin pointer to unaliased object

    19.    P (X2'Unrestricted_Access);         -- OK
    20. end;

However we can't catch all cases, so some cases just remain erroneous:

     1. with System; use System;
     2. procedure SliceUA is
     3.    type AF is access all String;
     4.
     5.    type A is access all String;
     6.    for A'Size use Standard'Address_Size;
     7.
     8.    procedure P (Arg : A) is
     9.    begin
    10.       if Arg'Length /= 6 then
    11.          raise Program_Error;
    12.       end if;
    13.    end P;
    14.
    15.    X : String := "hello world!";
    16.    Y : AF := X (7 .. 12)'Unrestricted_Access;
    17.
    18. begin
    19.    P (A (Y));
    20. end;

Here the conversion in the call on line 19 from a fat pointer to a
thin pointer is erroneous, and executing this program inevitably
raises Program_Error since the bounds get lost in the conversion.

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

2014-07-16  Robert Dewar  <dewar@adacore.com>

	* gnat_rm.texi: Document illegal case of Unrestricted_Access.
	* sem_attr.adb (Analyze_Access_Attribute): Set_Non_Aliased_Prefix
	where it applies.
	(Resolve_Attribute, case Access): Flag illegal Unrestricted_Access use.
	* sinfo.ads, sinfo.adb (Non_Aliased_Prefix): New flag.

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