[clang-format] Fixes indentation of inner text proto messages

Summary:
Consider the text proto:
```
message {
  sub { key: value }
}
```
Previously the first `{` was TT_Unknown, which caused the inner message to be
indented by the continuation width. This didn't happen for:
```
message {
  sub: { key: value }
}
```
This is because the code to mark the first `{` as a TT_DictLiteral was only
considering the case where it marches forward and reaches a `:`.

This patch updates this by looking not only for `:`, but also for `<` and `{`.

Reviewers: djasper

Reviewed By: djasper

Subscribers: klimek, cfe-commits

Differential Revision: https://reviews.llvm.org/D42500

llvm-svn: 323419