[Ada] Overlapping parameter is error not warning in Ada 2012 mode

[Arnaud Charlet <charlet at adacore dot com>]

This patch implements the required incompatible change in Ada 2012
that makes overlap of elementary parameters passed as OUT or IN OUT
an error (rather than a warning situation). It also implements the
debug flag -gnatd.E that converts this back to a warning.

The following program compiled in Ada 2012 mode gives:

     1. with Text_IO; use Text_IO;
     2. procedure OverlapWarn is
     3.    procedure OW (a, b : out Integer) is
     4.    begin
     5.       A := 3;
     6.       B := 3;
     7.    end;
     8.    X : Integer;
     9. begin
    10.    OW (X, X);
               |
        >>> writable actual for "a" overlaps with actual for "b"

    11.    Put_Line (X'Img);
    12. end OverlapWarn;

But if -gnatd.E is given in Ada 2012 mode, the error is a warning:

     1. with Text_IO; use Text_IO;
     2. procedure OverlapWarn is
     3.    procedure OW (a, b : out Integer) is
     4.    begin
     5.       A := 3;
     6.       B := 3;
     7.    end;
     8.    X : Integer;
     9. begin
    10.    OW (X, X);
               |
        >>> warning: writable actual for "a" overlaps with actual for "b"

    11.    Put_Line (X'Img);
    12. end OverlapWarn;

And the program can be executed, and outputs 3

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

2013-10-14  Robert Dewar  <dewar@adacore.com>

	* debug.adb: Document -gnatd.E.
	* gnat1drv.adb (Adjust_Global_Switches): Set Error_To_Warning
	if -gnatd.E set.
	* opt.ads (Error_To_Warning): New switch.
	* osint.adb: Minor reformatting.
	* sem_warn.adb (Warn_On_Overlapping_Actuals): Overlap is error
	in some cases in Ada 2012 mode (unless Error_To_Warning) is set.
	* sem_warn.ads (Warn_On_Overlapping_Actuals): Document error
	in Ada 2012 mode.

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