[clang-format] Fix parsing of msg{field}-style proto options

Summary:
This patch makes the `{` in `msg_field{field: OK}` in a proto option scope be
treated as an assignment operator. Previosly the added test case was formatted
as:
```
option (MyProto.options) = {
  field_a: OK
  field_b{field_c: OK} field_d: OKOKOK field_e: OK
}
```

Reviewers: djasper

Reviewed By: djasper

Subscribers: klimek, cfe-commits

Differential Revision: https://reviews.llvm.org/D34749

llvm-svn: 306672