[clang-format] Discourage breaks in submessage entries, hard rule

Summary:
Currently clang-format allows this for text protos:
```
submessage:
    { key: 'aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa' }
```
when it is under the column limit and when putting it all on one line exceeds the column limit.

This is not a very intuitive formatting, so I'd prefer having
```
submessage: {
  key: 'aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa'
}
```
instead, even if it takes one line more.

This patch prevents clang-format from inserting a break between `: {` and similar cases.

Reviewers: djasper, sammccall

Reviewed By: sammccall

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D48063

llvm-svn: 334517