Add support for specifying arbitrary --objects

[rcoh]

I recently added coverage testing for duchess which has a somewhat complicated setup of regular unit tests & and subproject containing ui tests.

This meant I had to go manually pick up all of the --object invocations that were needed which (I think) made it impossible to use cargo llvm-cov. Ideally, I'd be able to use a combination of https://github.com/taiki-e/cargo-llvm-cov?tab=readme-ov-file#get-coverage-of-external-tests and this additional configuration behavior to be able to use this tool directly.

Is there a better way of doing this that would have meant I could have used cargo llvm-cov directly without changes? Or is some sort of additional configuration required.

[taiki-e]

I think you can use LLVM_COV_FLAGS environment-variable, but I'm okay with adding a specific option for out-of-target-dir object files.