Remove emulation of hygiene with gensyms

Move `is_builtin` for `Mark` to a separate flag

It's so confusing to have everything having the same name, at least while refactoring.

When writing a struct literal in an expression that expects a block to
be started afterwards (like an `if` statement), do not suggest using the
same struct literal:

```
did you mean `S { /* fields * /}`?
```

Instead, suggest surrounding the expression with parentheses:

```
did you mean `(S { /* fields * /})`?
```

Fixes #50605

fixes #50970

No error checking or feature gating yet

This commit fixes recommending the `crate` prefix when migrating to 2018 for
paths that look like `use foo;` or `use {bar, baz}`

Closes #50660