AST: Clean up FriendObjectKind related decls

Allow the comments in the FriendObjectKind enumerator-list show up in
doxygen. Also, some small readability improvements in related functions.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@184657 91177308-0d34-0410-b5e6-96231b3b80d8