Split Objective-C keyword parsing to handle the '@' separately.

Because apparently, while designing the Objective-C language, somebody
thought it was a world-class idea to allow any white-space between the
'@' character and the subsequent keyword. With this fix, we now
correctly parse:
    @  dynamic
and:
    @
      selector
and:
    @"foo"
     "bar"
    @"mooze"
(This last one is 1 single string split over multiple lines.)
Wonderful, isn't it?

What we (and Clang) do not support, but what GCC supports is something
like:
   @"foo"@@  "bar"  @"mooze"  @@
which is equivalent to @"foobarmooze".

src/shared/cplusplus/Semantic.cpp

@@ -218,13 +218,13 @@ int Semantic::visibilityForAccessSpecifier(int tokenKind) const
 int Semantic::visibilityForObjCAccessSpecifier(int tokenKind) const
 {
     switch (tokenKind) {
-    case T_AT_PUBLIC:
+    case T_PUBLIC:
         return Symbol::Public;
-    case T_AT_PROTECTED:
+    case T_PROTECTED:
         return Symbol::Protected;
-    case T_AT_PRIVATE:
+    case T_PRIVATE:
         return Symbol::Private;
-    case T_AT_PACKAGE:
+    case T_PACKAGE:
         return Symbol::Package;
     default:
         return Symbol::Protected;