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

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

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

This fixes llvm.org/PR19731.