Infer the types of function/closure arguments when captured by an exit test.

[grynspan]

This PR adds the ability to infer the type of a parameter of a function or closure that encloses an exit test. For example, x here:

func f(x: Int) async {
  await #expect(processExitsWith: .failure) { [x] in
    ...
  }
}

This inference still fails if a parameter is shadowed by a variable with an incompatible type; we still need something like decltype() to solve for such cases. Still, being able to capture @Test function arguments with minimal ceremony is helpful:

@Test(arguments: 0 ..< 100) func f(i: Int) async {
  await #expect(exitsWith: .failure) { [i] in
    ...
  }
}

Also type inference for literals because "why not?"

Note

Exit test value capture remains an experimental feature.

Checklist:

• Code and documentation should follow the style of the Style Guide.
• If public symbols are renamed or modified, DocC references should be updated.

[grynspan]

@swift-ci test