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

[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

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@306406 91177308-0d34-0410-b5e6-96231b3b80d8