[Ada] Implement Valid_Scalars attribute (except for variant records)

[Arnaud Charlet <charlet at adacore dot com>]

This patch implements the new Valid_Scalars attribute (that tests all
scalar parts of an object including discriminabnts and subcomponents,
to ensure they are valid. All cases are handled (including multi-
dimensional arrays) except for variant records which will be
implemented in a separate step.

The following shows warnings that are generated (compiled with -gnatc,
 -gnatld7 -gnatj60)

     1. package ValidScalarsW is
     2.    type Ptr is access Integer;
     3.
     4.    type Rec is tagged record
     5.       A, B : Ptr;
     6.    end record;
     7.
     8.    type RecN is new Rec with record
     9.       X : Integer;
    10.    end record;
    11.
    12.    type Arr is array (1 .. 10) of Ptr;
    13.
    14.    V1 : Ptr;
    15.    V2 : Rec;
    16.    V3 : Rec'Class := V2;
    17.    V4 : Arr;
    18.
    19.    M1 : Boolean := V1'Valid_Scalars;
                           |
        >>> warning: attribute "Valid_Scalars" always True,
            no scalars to check

    20.    M2 : Boolean := V2'Valid_Scalars;
                           |
        >>> warning: attribute "Valid_Scalars" always True,
            no scalars to check

    21.    M3 : Boolean := V3'Valid_Scalars;
                           |
        >>> warning: attribute "Valid_Scalars" always True,
            no scalars to check

    22.    M4 : Boolean := V4'Valid_Scalars;
                           |
        >>> warning: attribute "Valid_Scalars" always True,
            no scalars to check

    23. end ValidScalarsW;

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

2012-04-02  Robert Dewar  <dewar@adacore.com>

	* einfo.adb (First_Component_Or_Discriminant) Now applies to
	all types with discriminants, not just records.
	* exp_attr.adb (Expand_N_Attribute): Add Scalar_Values handling
	for arrays, scalars and non-variant records.
	* sem_attr.adb (Analyze_Attribute): Handle Valid_Scalars
	* sem_attr.ads (Valid_Scalars): Update description
	* sem_util.ads, sem_util.adb (No_Scalar_Parts): New function.

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