Unbreak support for Buck2 tuple type syntax (#1509)

In Buck2:

* `()` is the type of the empty tuple (what Bazel calls `tuple[()]`)
* `(T,)` is the type of a 1-element tuple of T
* `(T, U)` or equivalently `(T, U,)` is the type of a 2-element tuple
type

etc.

This means we need to allow tuples of type expressions as type atoms.
(And we may as well allow arbitrary parenthesized type expressions as
type atoms, since Buck2 suppports those too.)

Fixes #1508.

## Buildtools PR checklist

- [X] The code in this PR is covered by unit/integration tests.
- [X] I have tested these changes and provide testing instructions
below.
- [ ] I have either responded to, or resolved all Gemini comments on the
PR.
- [X] I have read Google Eng Practices on [Small
Changes](https://google.github.io/eng-practices/review/developer/small-cls.html),
this PR either follows these guidelines or the description provides
reasoning for why they can not be followed.

@fmeum @dtolnay