`rustc --test lib.rs` when `#[crate_type = "rlib"]` produces unwanted *.bc file

[lilyball]

When compiling a crate that specifies rlib using --test, a spurious libname.bc file is created. This issue is new sometime in the past few days, and exists as of ca54ad8.

> ls
lib.rs
> cat lib.rs
#[crate_type = "rlib"];
> rustc --test lib.rs
> ls
lib     lib.bc  lib.rs