llvm-isel-fuzzer: Make buildable and testable without libFuzzer

This adds a dummy main so we can build and run the llvm-isel-fuzzer
functionality when we aren't building LLVM with coverage. The approach
here should serve as a template to stop in-tree fuzzers from
bitrotting (See llvm.org/pr34314).

Note that I'll probably move most of the logic in DummyISelFuzzer's
`main` to a library so it's easy to reuse it in other fuzz targets,
but I'm planning on doing that in a follow up that also consolidates
argument handling in our LLVMFuzzerInitialize implementations.

llvm-svn: 312338