compiletest test causes everything to rebuild

[ehuss]

If you run cargo test --test compile-test and then run it again, everything gets rebuilt (this requires the most recent nightly).

This is because of this line:

rust-clippy/tests/compile-test.rs

Line 99 in 0331b95

cfg.clean_rmeta();

This deletes all .rmeta files. With the recent changes to cargo for pipelining, cargo always generates rmeta files and expects them to not to be deleted.