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_parse/src/validate_attr.rs

@@ -29,7 +29,7 @@ pub fn check_attr(sess: &ParseSess, attr: &Attribute) {
         _ if let AttrArgs::Eq(..) = attr.get_normal_item().args => {
             // All key-value attributes are restricted to meta-item syntax.
             parse_meta(sess, attr)
-                .map_err(|mut err| {
+                .map_err(|err| {
                     err.emit();
                 })
                 .ok();
@@ -139,7 +139,7 @@ pub fn check_builtin_attribute(
 ) {
     match parse_meta(sess, attr) {
         Ok(meta) => check_builtin_meta_item(sess, &meta, attr.style, name, template),
-        Err(mut err) => {
+        Err(err) => {
             err.emit();
         }
     }
@@ -208,7 +208,7 @@ fn emit_malformed_attribute(
     } else {
         sess.dcx
             .struct_span_err(span, error_msg)
-            .span_suggestions(
+            .span_suggestions_mv(
                 span,
                 if suggestions.len() == 1 {
                     "must be of the form"