[Pitch] Adoption tooling for upcoming features

[anthonylatsis]

@cal

In theory you could have a third-party tool take the compiler output and automatically read, digest, and apply the given fix-its. Would it be possible for a third-party tool to consume these fix-its? From what I remember, fix-its don't get printed via standard out.

They do get printed with the LLVM formatter: -diagnostic-style llvm. But I think such tools should instead rely on -serialize-diagnostics-path (frontend flag) or -emit-module-serialize-diagnostics-path. Plus -print-diagnostic-groups (currently a hidden frontend flag) to filter out relevant diagnostics until we serialize the group.