Fix const items not being allowed to be called r#move or r#static

Because of an ambiguity with const closures, the parser needs to ensure
that for a const item, the `const` keyword isn't followed by a `move` or
`static` keyword, as that would indicate a const closure:

```rust
fn main() {
  const move // ...
}
```

This check did not take raw identifiers into account, therefore being
unable to distinguish between `const move` and `const r#move`. The
latter is obviously not a const closure, so it should be allowed as a
const item.

This fixes the check in the parser to only treat `const ...` as a const
closure if it's followed by the *proper keyword*, and not a raw
identifier.

Additionally, this adds a large test that tests for all raw identifiers in
all kinds of positions, including `const`, to prevent issues like this
one from occurring again.

compiler/rustc_span/src/symbol.rs

@@ -104,6 +104,8 @@ symbols! {
         Gen:                "gen", // >= 2024 Edition only
         Try:                "try", // >= 2018 Edition only
 
+        // NOTE: When adding new keywords, consider adding them to the ui/parser/raw/raw-idents.rs test.
+
         // "Lifetime keywords": regular keywords with a leading `'`.
         // Matching predicates: `is_any_keyword`
         UnderscoreLifetime: "'_",