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/parser/diagnostics.rs

@@ -896,7 +896,7 @@ impl<'a> Parser<'a> {
             let struct_expr = snapshot.parse_expr_struct(None, path, false);
             let block_tail = self.parse_block_tail(lo, s, AttemptLocalParseRecovery::No);
             return Some(match (struct_expr, block_tail) {
-                (Ok(expr), Err(mut err)) => {
+                (Ok(expr), Err(err)) => {
                     // We have encountered the following:
                     // fn foo() -> Foo {
                     //     field: value,
@@ -1218,7 +1218,7 @@ impl<'a> Parser<'a> {
                             "::",
                             Applicability::MaybeIncorrect,
                         )
-                        .emit();
+                        .emit_without_consuming();
                         match self.parse_expr() {
                             Ok(_) => {
                                 *expr = self.mk_expr_err(expr.span.to(self.prev_token.span));
@@ -2045,7 +2045,7 @@ impl<'a> Parser<'a> {
     ) -> P<Expr> {
         match result {
             Ok(x) => x,
-            Err(mut err) => {
+            Err(err) => {
                 err.emit();
                 // Recover from parse error, callers expect the closing delim to be consumed.
                 self.consume_block(delim, ConsumeClosingDelim::Yes);
@@ -2470,7 +2470,7 @@ impl<'a> Parser<'a> {
                     return Ok(true); // Continue
                 }
             }
-            Err(mut err) => {
+            Err(err) => {
                 args.push(arg);
                 // We will emit a more generic error later.
                 err.delay_as_bug();
@@ -2946,7 +2946,7 @@ impl<'a> Parser<'a> {
     }
 
     pub fn recover_diff_marker(&mut self) {
-        if let Err(mut err) = self.err_diff_marker() {
+        if let Err(err) = self.err_diff_marker() {
             err.emit();
             FatalError.raise();
         }