[clang-format] Support <>-style proto message fields

Summary:
This patch adds support for <>-style proto message fields inside proto options.
Previously these were wrongly treated as binary operators and as such were
working only by chance for a limited number of cases.

Reviewers: djasper

Reviewed By: djasper

Subscribers: cfe-commits, klimek

Differential Revision: https://reviews.llvm.org/D34621

llvm-svn: 306406