Provide config.toml flag (or so) to compile without deny(warnings)

[RalfJung]

It would sometimes be nice to run the compiler on a testcase even though there are currently warnings in the compiler. One way might be to have a config.toml flag that controls this.

Cc @eddyb

[eddyb]

cc @alexcrichton @Mark-Simulacrum

Maybe we can move the #![deny(warnings)] out of all the crates, and into rustbuild?

[Mark-Simulacrum]

I have a branch that fixes this based on the rustbuild-test branch to avoid rebasing. Will make a PR once the testing work lands.