[clang-format] Fix parsing of <>-style proto options

Summary:
This patch fixes the parsing of proto option fields like `option op = <...>`.
Previously the parser did not enter the right code path inside the angle braces,
causing the contents to be split into several unwrapped lines inside.

I'll just go ahead and commit this since it's a straightforward bugfix.

Reviewers: djasper, klimek

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D36217

llvm-svn: 309937