swift

365f0afa - Downgrade concurrency-related function type errors in existing code

Downgrade concurrency-related function type errors in existing code When in "existing" Swift code that is Swift 5.x and has not adopted concurrency, allow mismatches in function types that only involve ABI-neutral concurrency changes (e.g., adding `@Sendable` or removing a global actor) by downgrading the diagnostic to a warning. This improves the story for incremental adoption of concurrency in an existing code base. As part of this, generalize the facility for downgrading an error to a warning when performing diagnostics in the constraint solver, using the new diagnostic behavior limits rather than duplicating diagnostics.