What is the process to add a compiler flag?

[nikomatsakis]

Consider a PR like rust-lang/rust#69718, which exposes some LLVM feature for attaching cryptographic hashes of the sources. We don't have a documented process for what it takes to add a compiler flag like this. We've done it a few times informally -- I'm thinking of the sanitizer work, for example, and a few other cases (we should get make out a list). Let's write-up a procedure and post it to forge.

[nikomatsakis]

See also https://rust-lang.zulipchat.com/#narrow/stream/131828-t-compiler/topic/proposal.3A.20add.20cryptographic.20hashes.20to.20debuginfo.20.2369718