[clang-format] Support repeated field lists in protos

Summary:
This patch adds support for list initialization of proto repeated fields:
```
keys: [1, 2, 3]
```

Reviewers: djasper

Reviewed By: djasper

Subscribers: klimek, cfe-commits

Differential Revision: https://reviews.llvm.org/D43298

llvm-svn: 325252