Fix formatting of expressions containing ">>".

This gets turned into two ">" operators at the beginning in order to
simplify template parameter handling. Thus, we need a special case to
handle those two binary operators correctly.

With this patch, clang-format can now correctly handle cases like:
aaaaaa = aaaaaaa(aaaaaaa, // break
                 aaaaaa) >>
         bbbbbb;

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@182754 91177308-0d34-0410-b5e6-96231b3b80d8