Auto merge of #89933 - est31:let_else, r=michaelwoerister

Adopt let_else across the compiler

This performs a substitution of code following the pattern:

```
let <id> = if let <pat> = ... { identity } else { ... : ! };
```

To simplify it to:

```
let <pat> = ... { identity } else { ... : ! };
```

By adopting the `let_else` feature (cc #87335).

The PR also updates the syn crate because the currently used version of the crate doesn't support `let_else` syntax yet.

Note: Generally I'm the person who *removes* usages of unstable features from the compiler, not adds more usages of them, but in this instance I think it hopefully helps the feature get stabilized sooner and in a better state. I have written a [comment](https://github.com/rust-lang/rust/issues/87335#issuecomment-944846205) on the tracking issue about my experience and what I feel could be improved before stabilization of `let_else`.

compiler/rustc_resolve/src/diagnostics.rs

@@ -1471,9 +1471,7 @@ impl<'a, 'b> ImportResolver<'a, 'b> {
         module: ModuleOrUniformRoot<'b>,
         ident: Ident,
     ) -> Option<(Option<Suggestion>, Vec<String>)> {
-        let mut crate_module = if let ModuleOrUniformRoot::Module(module) = module {
-            module
-        } else {
+        let ModuleOrUniformRoot::Module(mut crate_module) = module else {
             return None;
         };
 

compiler/rustc_resolve/src/late/lifetimes.rs

@@ -1057,9 +1057,7 @@ impl<'a, 'tcx> Visitor<'tcx> for LifetimeContext<'a, 'tcx> {
                     match param.kind {
                         GenericParamKind::Lifetime { .. } => {
                             let (name, reg) = Region::early(&self.tcx.hir(), &mut index, &param);
-                            let def_id = if let Region::EarlyBound(_, def_id, _) = reg {
-                                def_id
-                            } else {
+                            let Region::EarlyBound(_, def_id, _) = reg else {
                                 bug!();
                             };
                             // We cannot predict what lifetimes are unused in opaque type.

compiler/rustc_resolve/src/lib.rs

@@ -15,6 +15,7 @@
 #![feature(crate_visibility_modifier)]
 #![feature(format_args_capture)]
 #![feature(iter_zip)]
+#![feature(let_else)]
 #![feature(never_type)]
 #![feature(nll)]
 #![recursion_limit = "256"]