fixup: Restore comment for TokenKey

This comment was originally on the namespace containing TokenKey and
KeywordStatus. I've only restored it to TokenKey because KeywordStatus
is used by the code that uses TokenKinds.def not TokenKinds.def itself.

Author: dsandersllvm

clang/include/clang/Basic/IdentifierTable.h

@@ -46,6 +46,7 @@ class LangOptions;
 class MultiKeywordSelector;
 class SourceLocation;
 
+/// Constants for TokenKinds.def
 enum TokenKey : unsigned {
   KEYC99 = 0x1,
   KEYCXX = 0x2,