[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

llvm-svn: 307261