[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