[clang-format] correctly format protobuf fields named "enum".

Summary: Similar to TypeScript, "enum" is not a reserved word.

Reviewers: krasimir, MyDeveloperDay

Reviewed By: MyDeveloperDay

Subscribers: MyDeveloperDay, cfe-commits

Tags: #clang, #clang-tools-extra

Differential Revision: https://reviews.llvm.org/D59629

Patch by: dchai (Donald Chai)

llvm-svn: 356833