Add `fn args_replace` and `fn get_program` to `ProcessBuilder`

`Executor::exec()` provides a `ProcessBuilder` argument representing compiler call and if someone wants to use it or modify it for their own purposes, they have to (partially) recreate it themselves.

This:
* makes the argument modification possible (via `args_replace`, analogous to `env_remove`)
* exposes the program that will be run (via `get_program`, in case `rustc` is overriden via wrapper etc.).

r? @alexcrichton

Allow to substitute executed program in `ProcessBuilder`

Extends #4424. Sorry for the noise, turns out it'd also be useful to substite the executed program (e.g. for rls, which needs to use a rustc shim itself) without having to copy over manually all parts of the `ProcessBuilder`.

While I'm at it, do you think this needs even more functionality, like being able to specify `None` for the `cwd` or `jobserver` (currently API exposes only ability to specify `Some(...)` as values for those)?

r? @alexcrichton