Rollup merge of #120221 - compiler-errors:statements-are-not-patterns, r=nnethercote

Don't make statement nonterminals match pattern nonterminals

Right now, the heuristic we use to check if a token may begin a pattern nonterminal falls back to `may_be_ident`:
https://github.com/rust-lang/rust/blob/ef71f1047e04438181d7cb925a833e2ada6ab390/compiler/rustc_parse/src/parser/nonterminal.rs#L21-L37

This has the unfortunate side effect that a `stmt` nonterminal eagerly matches against a `pat` nonterminal, leading to a parse error:
```rust
macro_rules! m {
    ($pat:pat) => {};
    ($stmt:stmt) => {};
}

macro_rules! m2 {
    ($stmt:stmt) => {
        m! { $stmt }
    };
}

m2! { let x = 1 }
```

This PR fixes it by more accurately reflecting the set of nonterminals that may begin a pattern nonterminal.

As a side-effect, I modified `Token::can_begin_pattern` to work correctly and used that in `Parser::nonterminal_may_begin_with`.

Author: matthiaskrgr