[Ada] Ignore_Pragma causes errors in the run-time system

[Arnaud Charlet <charlet at adacore dot com>]

This patch fixes a bug in which pragma Ignore_Pragma can cause errors in the
run-time system, if it applies to pragmas actually used in the run-time
system. Pragma Ignore_Pragma no longer applies to pragmas in the run-time
system.

The following test should compile quietly.

--  gnat.adc

pragma Ignore_Pragma(Import);


--  ignore_pragmas.adb

with Text_IO;
procedure Ignore_Pragmas is
begin
   null;
end Ignore_Pragmas;

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

2017-04-25  Bob Duff  <duff@adacore.com>

	* sem_util.ads, sem_util.adb (Should_Ignore_Pragma): New function
	that returns True when appropriate.
	* par-prag.adb, exp_prag.adb, sem_prag.adb: Do not ignore pragmas
	when compiling predefined files.
	* fname.ads, fname.adb (Is_Predefined_File_Name): Fix bug:
	"gnat.adc" should not be considered a predefined file name.
	That required (or at least encouraged) a lot of cleanup of global
	variable usage. We shouldn't be communicating information via
	the global name buffer.
	* bindgen.adb, errout.adb, fname-uf.adb, lib-load.adb, make.adb,
	* restrict.adb, sem_ch10.adb, sem_ch6.adb, sem_ch8.adb: Changes
	required by the above-mentioned cleanup.

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