[clang-format] Add space between a message field key and the opening bracket in proto...

[clang-format] Add space between a message field key and the opening bracket in proto messages

Summary:
This patch updates the formatting of message fields of type `a{...}` to `a {...}`
for proto messages.

Reviewers: djasper

Reviewed By: djasper

Subscribers: klimek, cfe-commits

Differential Revision: https://reviews.llvm.org/D35015

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@307261 91177308-0d34-0410-b5e6-96231b3b80d8