[Ada] Clean up scanner

This patch removes some obsolete code in the scanner and related files,
and corrects some comments. Tok_Special is used only by the
preprocessor, and uses only the two characters '#' and '$'.

It might be simpler to have a single flag indicating we're scanning for
preprocessing, instead of the Special_Characters array and the
End_Of_Line_Is_Token flag, but that's for another day.

gcc/ada/

	* scans.ads: Fix obsolete comments about Tok_Special, and give
	Special_Character a predicate assuring it is one of the two
	characters used in preprocessing.
	* scng.ads: Clean up comments.
	* scng.adb: Clean up handling of Tok_Special.  Remove comment
	about '@' (target_name), which doesn't seem very helpful.
	Set_Special_Character will now blow up if given anything other
	than '#' and '$', because of the predicate on Special_Character;
	it's not clear why it used to say "when others => null;".
	Remove Comment_Is_Token, which is not used.
	* scn.ads: Remove commented-out use clause.  Remove redundant
	comment.
	* ali-util.adb: Use "is null" for do-nothing procedures.
	* gprep.adb (Post_Scan): Use "is null".

1 files changed, 6 insertions, 13 deletions

diff --git a/gcc/ada/scans.ads b/gcc/ada/scans.ads
index d73dae4..dbe9e5a 100644
--- a/gcc/ada/scans.ads
+++ b/gcc/ada/scans.ads
@@ -210,15 +210,11 @@ package Scans is
 
       Tok_End_Of_Line,
       --  Represents an end of line. Not used during normal compilation scans
-      --  where end of line is ignored. Active for preprocessor scanning and
-      --  also when scanning project files (where it is needed because of ???)
+      --  where end of line is ignored. Active for preprocessor scanning.
 
       Tok_Special,
-      --  AI12-0125-03 : target name as abbreviation for LHS
-
-      --  Otherwise used only in preprocessor scanning (to represent one of
-      --  the characters '#', '$', '?', '@', '`', '\', '^', '~', or '_'. The
-      --  character value itself is stored in Scans.Special_Character.
+      --  Special character used by the preprocessor. The character itself is
+      --  stored in Special_Character below.
 
       No_Token);
       --  No_Token is used for initializing Token values to indicate that
@@ -466,12 +462,9 @@ package Scans is
    --  character found (i.e. a character that does not fit in Character or
    --  Wide_Character).
 
-   Special_Character : Character;
-   --  AI12-0125-03 : '@' as target name is handled elsewhere.
-   --  Valid only when Token = Tok_Special. Returns one of the characters
-   --  '#', '$', '?', '`', '\', '^', '~', or '_'.
-   --
-   --  Why only this set? What about wide characters???
+   subtype Special_Preprocessor_Character is Character with
+     Predicate => Special_Preprocessor_Character in '#' | '$';
+   Special_Character : Special_Preprocessor_Character;
 
    Comment_Id : Name_Id := No_Name;
    --  Valid only when Token = Tok_Comment. Store the string that follows