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`:
ef71f1047e/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`.
1fd0c71818