[clang-fuzzer] Allow building without coverage instrumentation.

Summary:
Compile with DummyClangFuzzer.cpp as entry point rather than
libFuzzer's main when coverage instrumentation is missing.

https://llvm.org/pr34314

Reviewers: kcc, bogner, vitalybuka

Reviewed By: vitalybuka

Subscribers: cfe-commits, mgorny

Differential Revision: https://reviews.llvm.org/D38642

llvm-svn: 315336