[NFC][Concurrency] Add a regression test for a bogus missing try diagnostic with async let.

[hborla]

func search(query: String) async throws -> [String] {
  let entities: [String] = []

  async let r = entities.filter { $0.contains(query) }.map { String($0) }
  return await r
}

The above example was reported as diagnosing error: reading 'async let' can throw but is not marked with 'try'. This has since been fixed on main, so this change is just adding a test case for the future.

Resolves rdar://96830163

[hborla]

@swift-ci please smoke test