[Ada] Clean up handling of assertions when disabled

[Arnaud Charlet <charlet at adacore dot com>]

This patch fixes a number of problems that arose from the handling
of assertions (more generally checks from pragma Checks). In
particular if a check was explicitly disabled with Check_Policy
then the argument was not analyzed leading to junk warnings.
The following should compile quietly with -gnatwa:

     1. pragma Check_Policy (Assertion, Disable);
     2. procedure Supcheck (X : Integer) is
     3.    Y : constant Integer := 32;
     4. begin
     5.    pragma Assert (X > Y);
     6.    null;
     7. end;

Previously there were warnings about X and Y not being referenced

In addition the following test:

     1. procedure BadconcA (I : Integer; S1 : String; S2 : String) is
     2. begin
     3.   pragma Assert (I > 0, S1 & S2);
     4.   null;
     5. end;

generates the following expanded code:

procedure badconca (i : integer; s1 : string; s2 : string) is
   subtype badconca__S2b is string (s2'first(1) .. s2'last(1));
   subtype badconca__S1b is string (s1'first(1) .. s1'last(1));
begin
   null;
   return;
end badconca;

A previous attempt at fixing this by messing with Expand_Concatenate
has been reversed (it caused difficulties and also was not 100%
successful, since it left around a junk with of System__Concat_2.

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

2013-04-11  Robert Dewar  <dewar@adacore.com>

	* exp_ch4.adb (Expand_Concatenate): Remove wrapping in
	expression-with-actions node. No longer needed given fix to
	sem_prag and caused loss of some useful warnings.
	* sem.ads: Minor reformatting.
	* sem_prag.adb (Check_Disabled): Removed, to be replaced by not
	Check_Enabled. These two routines were curiously incompatible
	causing confusion.
	(Analyze_Pragma, case Check): Make sure we do
	not expand the string argument if the check is disabled. Avoid
	use of Check_Disabled, which resulted in missing analysis in
	some cases.
	* sem_prag.ads (Check_Disabled): Removed, to be replaced by not
	Check_Enabled. These two routines were curiously incompatible
	causing confusion.

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