[clang-fuzzer] Add new fuzzer target for Objective-C

Summary:
- Similar to that of `clang-fuzzer` itself but instead only
  targets Objective-C source files via cc1

- Also adds an example corpus directory containing some
  input for Objective-C

Subscribers: mgorny, jfb, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D69171

llvm-svn: 375453