[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
git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@334517
91177308-0d34-0410-b5e6-
96231b3b80d8