[Ada] Implement new pragma Ignore_Pragma

[Arnaud Charlet <charlet at adacore dot com>]

This implements a new configuration pragma Ignore_Pragma (IDENTIFIER)
which causes any subsequent occurrences of pragma IDENTIFIER (...) to
be completely ignored. The following test program compiles as shown:

     1. procedure IgnorePR is
     2. begin
     3.    pragma Ignore_Pragma;
           |
        >>> wrong number of arguments for pragma "Ignore_Pragma"

     4.    pragma Ignore_Pragma ("abc");
                                 |
        >>> incorrect argument for pragma "Ignore_Pragma"

     5.    pragma Ignore_Pragma (Import);
     6.    pragma Import (RUBBISH);
     7. end;

Here lines 3 and 4 have invalid uses of the pragma, properly
flagged, but the junk pragma Import on line 6 is ignored
because of the correctly used Ignore_Pragma on line 5.

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

2015-05-12  Robert Dewar  <dewar@adacore.com>

	* exp_prag.adb (Expand_N_Pragma): Rewrite ignored pragma as
	Null statements.
	* namet.ads (Boolean3): Document this flag used for Ignore_Pragma.
	* par-prag.adb (Prag): Implement Ignore_Pragma.
	* sem_prag.adb: Implement Ignore_Pragma.
	* snames.ads-tmpl: Add entries for pragma Ignore_Pragma.

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