[Ada] Improve and unify warning machinery for address clauses

[Arnaud Charlet <charlet at adacore dot com>]

This change moves the rest of the warning machinery for address clauses to
Validate_Address_Clauses, ensuring that all the variants are issued from it.
This affects only absolute address clauses in practice, i.e. address clauses
of the form for I'Address use To_Address (16#XXXX_XXXX#) and variants thereof.

This automatically brings a couple of improvements: warnings are more accurate
because they take into account the final alignment set by the back-end and they
catch more cases because the back-end sets the alignment of every single type
and object in the program.  The warning also prints the alignment value now.

The following code gives an example of the warnings:

pragma Unsuppress (Alignment_Check);


     1. with System.Storage_Elements; use System.Storage_Elements;
     2.
     3. package P is
     4.
     5.   I : Integer;
     6.   for I'Address use To_Address (16#7FFF_0001#); -- warning
          |
        >>> warning: specified address for "I" is inconsistent with alignment
        >>> warning: program execution may be erroneous (RM 13.3(27))
        >>> warning: alignment of "I" is 4

     7.
     8.   type Rec is record
     9.     I : Integer;
    10.   end record;
    11.
    12.   R1 : Rec;
    13.   for R1'Address use To_Address (16#7FFF_0001#); -- warning
          |
        >>> warning: specified address for "R1" is inconsistent with alignment
        >>> warning: program execution may be erroneous (RM 13.3(27))
        >>> warning: alignment of "R1" is 4

    14.
    15.   C : constant System.Address := To_Address (16#7FFF_0001#); -- warning
    16.
    17.   R2 : Rec;
    18.   for R2'Address use C;
          |
        >>> warning: specified address for "R2" is inconsistent with alignment
        >>> warning: program execution may be erroneous (RM 13.3(27))
        >>> warning: alignment of "R2" is 4

    19.
    20.   R3 : Rec;
    21.   for R3'Address use To_Address (16#7FFF_0004#); -- no warning
    22.
    23. end P;

 23 lines: No errors, 9 warnings

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

2016-06-22  Eric Botcazou  <ebotcazou@adacore.com>

	* sem_util.ads (Address_Value): Declare new function.
	* sem_util.adb (Address_Value): New function extracted
	unmodified from Apply_Address_Clause_Check, which returns the
	underlying value of the expression of an address clause.
	* checks.adb (Compile_Time_Bad_Alignment): Delete.
	(Apply_Address_Clause_Check): Call Address_Value on
	the expression.  Do not issue the main warning here and
	issue the secondary warning only when the value of the
	expression is not known at compile time.
	* sem_ch13.adb (Address_Clause_Check_Record): Add A component and
	adjust the description.
	(Analyze_Attribute_Definition_Clause): In the case
	of an address, move up the code creating an entry in the table of
	address clauses.  Also create an entry for an absolute address.
	(Validate_Address_Clauses): Issue the warning for absolute
	addresses here too.  Tweak condition associated with overlays
	for consistency.

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