[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