clang-format: [Proto] Change formatting text-formatted options.

Before:
  optional Type type = 1 [(mutate_options) = {vital : true
                                              abc : false}];

After:
  optional Type type = 1 [(mutate_options) = {
    vital : true
    abc : false
  }];

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@220679 91177308-0d34-0410-b5e6-96231b3b80d8