[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

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@375453 91177308-0d34-0410-b5e6-96231b3b80d8