Rollup merge of #33336 - birkenfeld:issue-27361, r=sfackler

parser: do not try to continue with `unsafe` on foreign fns

The changed line makes it look like `unsafe` is allowed, but the first statement of `parse_item_foreign_fn` is:

```
self.expect_keyword(keywords::Fn)?;
```

So we get the strange "expected one of `fn`, `pub`, `static`, or `unsafe`, found `unsafe`".

Fixes: #27361

diff --cc src/libsyntax/parse/parser.rs
Simple merge