Allow suppression of cargo warning instructions in output

[scootermon]

Right now cc allows for the suppression of specific cargo instructions using .cargo_metdata(false), but this doesn't affect the output of cargo:warning= lines.
I'm working on a bindings library which uses compilation tests to determine whether certain features are supported or not. CC already almost fully supports this use-case¹, the only issue being that the warnings and errors show up as though they're part of the "main compilation".

Since I belive this feature is unrelated to the behaviour of cargo_metadata, how would you feel about adding a .cargo_warnings(false) setting that allows us to disable the warnings?
I'm of course happy to contribute the implementation.

Footnotes

1. Shoutout to Expose function compile to an object file #912, which will also help here because it helps remove an otherwise unnecessary step from the compilation tests ↩

[NobodyXu]

Yes I'm willing to review a PR disabling the warnings.