[6.2] Add (hidden) synchronous overloads of #expect(throws:). (#1179)

- **Explanation**: Suppress some new/spurious warnings from the compiler
in Swift 6.2 when using `#expect(throws:)` with a synchronous closure
that calls a `noasync` function.
- **Scope**: Testing errors thrown from synchronous code.
- **Issues**: #1177, rdar://149299786
- **Original PRs**: #1178
- **Risk**: No obvious risk.
- **Testing**: Existing test functions show the issue and that it's been
resolved.
- **Reviewers**: @stmontgomery @briancroom

Author: grynspan