[cfe-dev] [RFC] Refactor Clang: move frontend/driver/diagnostics code to LLVM

[comex via cfe-dev]

While this is a different area of the codebase, another thing that
would benefit greatly from being moved out of Clang is function call
ABI handling.  Currently, that handling is split awkwardly between
Clang and LLVM proper, forcing frontends that implement C FFI to
either recreate the Clang parts themselves (like Rust does), depend on
Clang (like Swift does), or live with FFI just not working with some
function signatures.  I'm not sure what Flang currently does, but my
understanding is that Flang does support C FFI, so it would probably
benefit from this as well.  Just something to consider. :)