[clang-format] Fix text proto extension scope opening detection

Summary:
This fixes the detection of scope openers in text proto extensions; previously
they were not detected correctly leading to instances like:
```
msg {
  [aa.bb
] {
key: value
}
}
```

Subscribers: klimek, cfe-commits

Differential Revision: https://reviews.llvm.org/D43469

llvm-svn: 325513