Reduce capabilities of Diagnostic.

Currently many diagnostic modifier methods are available on both
`Diagnostic` and `DiagnosticBuilder`. This commit removes most of them
from `Diagnostic`. To minimize the diff size, it keeps them within
`diagnostic.rs` but changes the surrounding `impl Diagnostic` block to
`impl DiagnosticBuilder`. (I intend to move things around later, to give
a more sensible code layout.)

`Diagnostic` keeps a few methods that it still needs, like `sub`,
`arg`, and `replace_args`.

The `forward!` macro, which defined two additional methods per call
(e.g. `note` and `with_note`), is replaced by the `with_fn!` macro,
which defines one additional method per call (e.g. `with_note`). It's
now also only used when necessary -- not all modifier methods currently
need a `with_*` form. (New ones can be easily added as necessary.)

All this also requires changing `trait AddToDiagnostic` so its methods
take `DiagnosticBuilder` instead of `Diagnostic`, which leads to many
mechanical changes. `SubdiagnosticMessageOp` gains a type parameter `G`.

There are three subdiagnostics -- `DelayedAtWithoutNewline`,
`DelayedAtWithNewline`, and `InvalidFlushedDelayedDiagnosticLevel` --
that are created within the diagnostics machinery and appended to
external diagnostics. These are handled at the `Diagnostic` level, which
means it's now hard to construct them via `derive(Diagnostic)`, so
instead we construct them by hand. This has no effect on what they look
like when printed.

There are lots of new `allow` markers for `untranslatable_diagnostics`
and `diagnostics_outside_of_impl`. This is because
`#[rustc_lint_diagnostics]` annotations were present on the `Diagnostic`
modifier methods, but missing from the `DiagnosticBuilder` modifier
methods. They're now present.

compiler/rustc_macros/src/diagnostics/subdiagnostic.rs

@@ -87,9 +87,13 @@ impl SubdiagnosticDeriveBuilder {
         let f = &self.f;
         let ret = structure.gen_impl(quote! {
             gen impl rustc_errors::AddToDiagnostic for @Self {
-                fn add_to_diagnostic_with<__F>(self, #diag: &mut rustc_errors::Diagnostic, #f: __F)
-                where
-                    __F: rustc_errors::SubdiagnosticMessageOp,
+                fn add_to_diagnostic_with<__G, __F>(
+                    self,
+                    #diag: &mut rustc_errors::DiagnosticBuilder<'_, __G>,
+                    #f: __F
+                ) where
+                    __G: rustc_errors::EmissionGuarantee,
+                    __F: rustc_errors::SubdiagnosticMessageOp<__G>,
                 {
                     #implementation
                 }