Introducing single for loop into clang_proto_fuzzer

Summary:
Created a new protobuf and protobuf-to-C++ "converter" that wraps the entire C++ code in a single for loop.
  - Slightly changed cxx_proto.proto -> cxx_loop_proto.proto
  - Made some changes to proto_to_cxx files to handle the new kind of protobuf
  - Created ExampleClangLoopProtoFuzzer to test new protobuf and "converter"

Patch by Emmett Neyman

Reviewers: kcc, vitalybuka, morehouse

Reviewed By: vitalybuka, morehouse

Subscribers: mgorny, llvm-commits, cfe-commits

Differential Revision: https://reviews.llvm.org/D47843

llvm-svn: 334216