Reorder diagnostics API

[nnethercote]

The totally random ordering of diagnostic methods in DiagCtxt has been low-key driving me crazy for a while now.

r? @compiler-errors

[nnethercote]

Here's the API summary table I've been using for a while now, which I used to guide this PR.

func                        args     ret          create/struct internals
----                    -----------  ---          -----------------------
{struct_,}bug               msg      DB<BA> / !   DB::new(BugAbort)
{struct_,}span_bug      sp, msg      DB<BA> / !   struct_bug + with_span
{create,emit}_bug           err      DB<BA> / !   .into_diagnostic

{struct_,}fatal             msg      DB<FA> / !   DB::new(FatalAbort)
{struct_,}span_fatal    sp, msg      DB<FA> / !   struct_fatal + with_span
{create,emit}_fatal         err      DB<FA> / !   .into_diagnostic

{create,emit}_almost_fatal  err      DB<FE> / FE  .into_diagnostic

{struct_,}err               msg      DB<>   / EG  DB::new(Error)
{struct_,}span_err      sp, msg      DB<>   / EG  struct_err + with_span
{create,emit}_err           err      DB<>   / EG  .into_diagnostic 

  delayed_bug               msg      -      / EG  DB::new(DelayedBug)) [*]
  span_delayed_bug      sp, msg      -      / EG  DB::new(DelayedBug) + with_span [*]

  good_path_delayed_bug     msg      -      / ()  DB::new(GoodPathDelayedBug) + emit [*]
                                                                 
{struct_,}warn              msg      DB<()> / ()  DB::new(Warning)
{struct_,}span_warn     sp, msg      DB<()> / ()  struct_warn + with_span
{create,emit}_warn          err      DB<()> / ()  .into_diagnostic 

{struct_,}note              msg      DB<()> / ()  DB::new(Note)  
{struct_,}span_note     sp, msg      DB<()> / ()  struct_note + with_span
{create,emit}_note          err      DB<()> / ()  .into_diagnostic 

struct_help                 msg      DB<()>       DB::new(Help) 

struct_allow                msg      DB<()>       DB::new(Allow) 

struct_expect               msg, id  DB<()>       DB::new(Expect(id))

[compiler-errors]

@bors r+ rollup

[bors]

📌 Commit d1920a7 has been approved by compiler-errors

It is now in the queue for this repository.

[nnethercote]

I fixed the conflicts.

@bors r=compiler-errors

[bors]

📌 Commit b7b6ebc has been approved by compiler-errors

It is now in the queue for this repository.