fuzz_mutator seems to not work with stripping dead code by default

[0xalpharush]

Using the package from crates prior to #394:

$ cargo fuzz --version
cargo-fuzz 0.12.0
$ git clone [email protected]:rust-fuzz/libfuzzer.git && cd libfuzzer/example_mutator
$ cargo fuzz run boom  
...
thread '<unnamed>' panicked at
...

If I install from git cargo install --git https://github.com/rust-fuzz/cargo-fuzz, it not longer panics nearly instantly (finds an input that is prefixed with "boom"). I suspect this is because fuzz_mutator is stripped due to #394. That said, I did notice stripping dead code fixed errors llvm-cov produced when I was trying to generating coverage reports.

More info:
If I compile the example in a standalone project without cargo-fuzz, I can reproduce the behavior more clearly.

$ cargo new example_mutator && cargo add flate2 libfuzzer-sys && mv ~/libfuzzer/example_mutator/fuzz_targets/boom.rs ~/example_mutator/main.rs
$ cargo +nightly rustc -- \                                      
    -C passes='sancov-module' \
    -C llvm-args='-sanitizer-coverage-level=4' \
    -C llvm-args='-sanitizer-coverage-inline-8bit-counters' \
    -C link-dead-code

Next, add a panic!(); to fuzz_mutator! and run ./target/debug/example_mutator. I get a crash at the inserted panic.

If I remove -C link-dead-code, the panic isn't reached.