[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".