clang-format: Don't break in the middle of ">>".

Before:
  zzzzzzzzzz = bbbbbbbbbbbbbbbbb >
               > aaaaaaaaaaaaaaaa(aaaaaaaaaaaaaaaaaaaaaaaaaa);

After:
  zzzzzzzzzz
      = bbbbbbbbbbbbbbbbb
        >> aaaaaaaaaaaaaaaa(aaaaaaaaaaaaaaaaaaaaaaaaaa);

This fixes llvm.org/PR19731.

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@208672 91177308-0d34-0410-b5e6-96231b3b80d8