[Ada] Clean up handling of pack, component size, aliased/atomic components

This patch cleans up the handling of pragma pack, component size clauses
and aliased/atomic components, to catch remaining illegal cases for Ada
2012, and also warn on some redundant packs missed before (now all such
warnings are under control of -gnatwr):

The following is compiled with -gnatwr -gnatj60 -gnatld7

     1. package packcsac is
     2.    type r1 is array (1 .. 3) of boolean;
     3.    pragma Atomic_Components (r1);
     4.    for r1'Component_Size use 2;
           |
        >>> incorrect component size for atomic components,
            only allowed value is 8

     5.
     6.    type r2 is array (1 .. 3) of boolean;
     7.    pragma Atomic_Components (r2);
     8.
     9.    type r3 is array (1 .. 3) of boolean;
    10.    for r3'Component_Size use 2;
           |
        >>> incorrect component size for atomic components,
            only allowed value is 8

    11.    pragma Atomic_Components (r3);
    12.
    13.    type r4 is array (1 .. 3) of boolean;
    14.    pragma Pack (r4);
           |
        >>> warning: pragma Pack for "r4" ignored, explicit
            component size given at line 15

    15.    for r4'Component_Size use 2;
    16.
    17.    type r6 is array (1 .. 3) of boolean;
    18.    for r6'Component_Size use 2;
    19.    pragma Pack (r6);
           |
        >>> warning: pragma Pack for "r6" ignored, explicit
            component size given at line 18

    20.
    21.    type r7 is array (1 .. 3) of boolean;
    22.    pragma Atomic_Components (r7);
    23.    pragma Pack (r7);
           |
        >>> cannot pack atomic components

    24.
    25.    type r8 is array (1 .. 3) of boolean;
    26.    pragma Pack (r8);
           |
        >>> cannot pack atomic components

    27.    pragma Atomic_Components (r8);
    28.
    29.    type R9 is array (1 .. 32) of boolean;
    30.    pragma Pack (R9);
    31.    pragma Atomic (R9);
           |
        >>> warning: non-atomic components of type "r9" may
            not be accessible by separate tasks, because of
            pragma Pack at line 30

    32.
    33.    type R10 is array (1 .. 32) of boolean;
    34.    pragma Atomic (R10);
           |
        >>> warning: non-atomic components of type "r10"
            may not be accessible by separate tasks,
            because of pragma Pack at line 35

    35.    pragma Pack (R10);
    36.
    37.    type R11 is array (1 .. 32) of boolean;
    38.    for R11'Component_Size use 1;
    39.    pragma Atomic (R11);
           |
        >>> warning: non-atomic components of type "r11"
            may not be accessible by separate tasks,
            because of component size clause at line 38

    40.
    41.    type R12 is array (1 .. 32) of boolean;
    42.    pragma Atomic (R12);
           |
        >>> warning: non-atomic components of type "r12"
            may not be accessible by separate tasks,
            because of component size clause at line 43

    43.    for R12'Component_Size use 1;
    44.
    45.    type R13 is array (1 .. 4) of Character;
    46.    pragma Atomic (R13);
    47.    for R13'Component_Size use 8;
    48. end;

This patch also corrects an error introduced by a previous recent
patch in this area which caused a regression on handling of
aliased components. The following must compile clean:

package Pkg is
  subtype SEM_OP_FLAGS_TYPE is Interfaces.C.short;

  type SEM_INDEX_TYPE is (VALUE, COUNT, LOCK);

    for SEM_INDEX_TYPE'SIZE use 16;

  type SEMBUF_TYPE is
    record
      SEM_NUM : SEM_INDEX_TYPE     := VALUE;  -- semaphore number
      SEM_OP  : Interfaces.C.short := 0;  -- semaphore operation value
      SEM_FLG : SEM_OP_FLAGS_TYPE  := 0;  -- operation flags
    end record;
    for SEMBUF_TYPE'SIZE use 48;

  type SEMOPS_TYPE is array (POSITIVE range <>) of aliased SEMBUF_TYPE;
    for SEMOPS_TYPE'COMPONENT_SIZE use 48;
end Pkg;

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

2010-10-08  Robert Dewar  <dewar@adacore.com>

	* freeze.adb (Freeze_Entity): For array case, move some processing for
	pragma Pack, Component_Size clause and atomic/volatile components here
	instead of trying to do the job in Sem_Ch13 and Freeze.
	* layout.adb: Use new Addressable function
	* sem_ch13.adb (Analyze_Attribute_Representation_Clause, case
	Component_Size): Move some handling to freeze point in
	Freeze.Freeze_Entity.
	* sem_prag.adb (Analyze_pragma, case Pack): Move some handling to
	freeze point in Freese.Freeze_Entity.
	* sem_util.ads, sem_util.adb (Addressable): New function.

[Ada] Clean up handling of pack, component size, aliased/atomic components

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