[clang-format] Support text proto extensions

Summary:
This adds support for text proto extensions, like:
```
msg {
  [type.type/ext] {
    key: value
  }
}
```

Reviewers: djasper

Reviewed By: djasper

Subscribers: klimek, cfe-commits

Differential Revision: https://reviews.llvm.org/D43180

llvm-svn: 324995
diff --git a/clang/lib/Format/ContinuationIndenter.cpp b/clang/lib/Format/ContinuationIndenter.cpp
index 64d8bb7..d28254d 100644
--- a/clang/lib/Format/ContinuationIndenter.cpp
+++ b/clang/lib/Format/ContinuationIndenter.cpp
@@ -943,6 +943,8 @@
   if (Previous.is(tok::r_paren) && !Current.isBinaryOperator() &&
       !Current.isOneOf(tok::colon, tok::comment))
     return ContinuationIndent;
+  if (Current.is(TT_ProtoExtensionLSquare))
+    return State.Stack.back().Indent;
   if (State.Stack.back().Indent == State.FirstIndent && PreviousNonComment &&
       PreviousNonComment->isNot(tok::r_brace))
     // Ensure that we fall back to the continuation indent width instead of