Add support for revisions without rustc

[DaniPopes]

Currently specifying the directive @revisions <revision>... implicitly appends --cfg=<revision> -Cextra-filename=<revision> to the compile flags, assuming the command is a rustc driver.

It would be nice to be able to use this without a rustc driver for running a test multiple times with different configurations and different output files.

I was thinking to add a simple config flag that turns the implicit compile flags off, since AFAICT this is the only implicit behavior of the directive. If this is acceptable I can open a PR to implement this.

[oli-obk]

Hmm... revisions are pretty special, but maybe we can still make them a Flag.

If we can't, we make the "command line flag adding" a Flag that you can either not set at all, or create your own version of if you want it to set env vars or different flags instead. The apply function can query the revision from the status emitter