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_parse/src/parser/mod.rs

@@ -754,9 +754,9 @@ impl<'a> Parser<'a> {
         self.is_keyword_ahead(0, &[kw::Const])
             && self.look_ahead(1, |t| match &t.kind {
                 // async closures do not work with const closures, so we do not parse that here.
-                token::Ident(kw::Move | kw::Static, _) | token::OrOr | token::BinOp(token::Or) => {
-                    true
-                }
+                token::Ident(kw::Move | kw::Static, IdentIsRaw::No)
+                | token::OrOr
+                | token::BinOp(token::Or) => true,
                 _ => false,
             })
     }