Rollup merge of #134274 - fmease:amp-raw-is-a-normal-borrow, r=Noratrieb

Add check-pass test for `&raw`

`&raw` denotes a normal/non-raw borrow of the path `raw`, not the start of raw borrow since it's not followed by either `const` or `mut`. Ensure this (and variants) will never regress!

When I saw the open diagnostic issue https://github.com/rust-lang/rust/issues/133231 (better parse error (recovery) on `&raw <expr>`), it made me think that we have to make sure that we will never commit too early/overzealously(†) when encountering the sequence `&raw`, even during parse error recovery!

Modifying the parser to eagerly treat `&raw` as the start of a raw borrow expr only lead to a single UI test failing, namely [tests/ui/enum-discriminant/ptr_niche.rs](https://github.com/rust-lang/rust/blob/4847d6a9d07d4be9ba3196f6ad444af2d7bdde72/tests/ui/enum-discriminant/ptr_niche.rs). However, this is just coincidental — it didn't *intentionally* test this edge case of the grammar.

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†: With "eager" I mean something like:

```patch
diff --git a/compiler/rustc_parse/src/parser/expr.rs b/compiler/rustc_parse/src/parser/expr.rs
index 0904a42d8a4..68d690fd602 100644
--- a/compiler/rustc_parse/src/parser/expr.rs
+++ b/compiler/rustc_parse/src/parser/expr.rs
`@@` -873,11 +873,16 `@@` fn error_remove_borrow_lifetime(&self, span: Span, lt_span: Span) {

     /// Parse `mut?` or `raw [ const | mut ]`.
     fn parse_borrow_modifiers(&mut self) -> (ast::BorrowKind, ast::Mutability) {
-        if self.check_keyword(kw::Raw) && self.look_ahead(1, Token::is_mutability) {
+        if self.eat_keyword(kw::Raw) {
             // `raw [ const | mut ]`.
-            let found_raw = self.eat_keyword(kw::Raw);
-            assert!(found_raw);
-            let mutability = self.parse_const_or_mut().unwrap();
+            let mutability = self.parse_const_or_mut().unwrap_or_else(|| {
+                let span = self.prev_token.span;
+                self.dcx().emit_err(ExpectedMutOrConstInRawBorrowExpr {
+                    span,
+                    after_ampersand: span.shrink_to_hi(),
+                });
+                ast::Mutability::Not
+            });
             (ast::BorrowKind::Raw, mutability)
         } else {
             // `mut?`
```

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r? compiler

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