Make DiagnosticBuilder::emit consuming.

This works for most of its call sites. This is nice, because `emit` very
much makes sense as a consuming operation -- indeed,
`DiagnosticBuilderState` exists to ensure no diagnostic is emitted
twice, but it uses runtime checks.

For the small number of call sites where a consuming emit doesn't work,
the commit adds `DiagnosticBuilder::emit_without_consuming`. (This will
be removed in subsequent commits.)

Likewise, `emit_unless` becomes consuming. And `delay_as_bug` becomes
consuming, while `delay_as_bug_without_consuming` is added (which will
also be removed in subsequent commits.)

All this requires significant changes to `DiagnosticBuilder`'s chaining
methods. Currently `DiagnosticBuilder` method chaining uses a
non-consuming `&mut self -> &mut Self` style, which allows chaining to
be used when the chain ends in `emit()`, like so:
```
    struct_err(msg).span(span).emit();
```
But it doesn't work when producing a `DiagnosticBuilder` value,
requiring this:
```
    let mut err = self.struct_err(msg);
    err.span(span);
    err
```
This style of chaining won't work with consuming `emit` though. For
that, we need to use to a `self -> Self` style. That also would allow
`DiagnosticBuilder` production to be chained, e.g.:
```
    self.struct_err(msg).span(span)
```
However, removing the `&mut self -> &mut Self` style would require that
individual modifications of a `DiagnosticBuilder` go from this:
```
    err.span(span);
```
to this:
```
    err = err.span(span);
```
There are *many* such places. I have a high tolerance for tedious
refactorings, but even I gave up after a long time trying to convert
them all.

Instead, this commit has it both ways: the existing `&mut self -> Self`
chaining methods are kept, and new `self -> Self` chaining methods are
added, all of which have a `_mv` suffix (short for "move"). Changes to
the existing `forward!` macro lets this happen with very little
additional boilerplate code. I chose to add the suffix to the new
chaining methods rather than the existing ones, because the number of
changes required is much smaller that way.

This doubled chainging is a bit clumsy, but I think it is worthwhile
because it allows a *lot* of good things to subsequently happen. In this
commit, there are many `mut` qualifiers removed in places where
diagnostics are emitted without being modified. In subsequent commits:
- chaining can be used more, making the code more concise;
- more use of chaining also permits the removal of redundant diagnostic
  APIs like `struct_err_with_code`, which can be replaced easily with
  `struct_err` + `code_mv`;
- `emit_without_diagnostic` can be removed, which simplifies a lot of
  machinery, removing the need for `DiagnosticBuilderState`.

compiler/rustc_hir_analysis/src/coherence/mod.rs

@@ -181,7 +181,7 @@ fn check_object_overlap<'tcx>(
                         trait_ref.self_ty(),
                         tcx.def_path_str(trait_def_id)
                     )
-                    .span_label(
+                    .span_label_mv(
                         span,
                         format!(
                             "`{}` automatically implements trait `{}`",

compiler/rustc_hir_analysis/src/coherence/unsafety.rs

@@ -25,7 +25,7 @@ pub(super) fn check_item(tcx: TyCtxt<'_>, def_id: LocalDefId) {
                     "implementing the trait `{}` is not unsafe",
                     trait_ref.print_trait_sugared()
                 )
-                .span_suggestion_verbose(
+                .span_suggestion_verbose_mv(
                     item.span.with_hi(item.span.lo() + rustc_span::BytePos(7)),
                     "remove `unsafe` from this trait implementation",
                     "",
@@ -42,13 +42,13 @@ pub(super) fn check_item(tcx: TyCtxt<'_>, def_id: LocalDefId) {
                     "the trait `{}` requires an `unsafe impl` declaration",
                     trait_ref.print_trait_sugared()
                 )
-                .note(format!(
+                .note_mv(format!(
                     "the trait `{}` enforces invariants that the compiler can't check. \
                     Review the trait documentation and make sure this implementation \
                     upholds those invariants before adding the `unsafe` keyword",
                     trait_ref.print_trait_sugared()
                 ))
-                .span_suggestion_verbose(
+                .span_suggestion_verbose_mv(
                     item.span.shrink_to_lo(),
                     "add `unsafe` to this trait implementation",
                     "unsafe ",
@@ -65,13 +65,13 @@ pub(super) fn check_item(tcx: TyCtxt<'_>, def_id: LocalDefId) {
                     "requires an `unsafe impl` declaration due to `#[{}]` attribute",
                     attr_name
                 )
-                .note(format!(
+                .note_mv(format!(
                     "the trait `{}` enforces invariants that the compiler can't check. \
                     Review the trait documentation and make sure this implementation \
                     upholds those invariants before adding the `unsafe` keyword",
                     trait_ref.print_trait_sugared()
                 ))
-                .span_suggestion_verbose(
+                .span_suggestion_verbose_mv(
                     item.span.shrink_to_lo(),
                     "add `unsafe` to this trait implementation",
                     "unsafe ",