[clang-format] Recognize percents as format specifiers in protos

Summary:
Frequently, a percent in protos denotes a formatting specifier for string replacement.
Thus it is desirable to keep the percent together with what follows after it.

Reviewers: djasper

Reviewed By: djasper

Subscribers: klimek, cfe-commits

Differential Revision: https://reviews.llvm.org/D43294

llvm-svn: 325159