clang-format: Add option to break before non-assignment operators.

This will allow:
  int aaaaaaaaaaaaaa =
      bbbbbbbbbbbbbb
      + ccccccccccccccc;

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@217757 91177308-0d34-0410-b5e6-96231b3b80d8