[clang-format] text protos: put entries on separate lines if there is a submessage

Summary:
This patch updates clang-format text protos to put entries of a submessage into separate lines if the submessage contains at least two entries and contains at least one submessage entry.

For example, the entries here are kept on separate lines even if putting them on a single line would be under the column limit:
```
message: {
  entry: 1
  submessage: { key: value }
}
```

Messages containing a single submessage or several scalar entries can still be put on one line if they fit:
```
message { submessage { key: value } }
message { x: 1 y: 2 z: 3 }
```

Reviewers: sammccall

Reviewed By: sammccall

Subscribers: klimek, cfe-commits

Differential Revision: https://reviews.llvm.org/D46757

llvm-svn: 334401