Issue Description:

I have a test which won't compile with the Swift 5 compiler (2019-02-06 snapshot), both in Swift 5 and Swift 4.2 modes:

XCTAssertEqual(
    Set(fetchedRecords.map { $0.name! }),
    Set([record1, record2].map { $0.name! }))

The compiler emits the following error:

/Users/groue/Documents/git/groue/GRDB.swift/Tests/GRDBTests/RecordPrimaryKeyNoneTests.swift:155:106: error: cannot assign through '!': '$0' is immutable
                XCTAssertEqual(Set(fetchedRecords.map { $0.name! }), Set([record1, record2].map { $0.name! }))
                                                                                                  ~~     ^

The error vanishes when the code is rewritten in the equivalent form below:

let fetchedNames = Set(fetchedRecords.map { $0.name! })
let expectedNames = Set([record1, record2].map { $0.name! })
XCTAssertEqual(fetchedNames, expectedNames)

For information, the record1, record2 variables are of type `Item`, and the fetchedRecords variable is of type `[Item]`:

private class Item: Record {
    var name: String?
    ...
}

To reproduce:

1. Clone https://github.com/groue/GRDB.swift.git
2. Checkout the 493575a47bc7feffecd91bdd411358e15f92e118 commit
3. Open GRDB.xcworkspace
4. Select the GRDBOSX scheme
5. Build tests with the 2019-02-06 snapshot > there is an error :(
6. Build tests with Xcode 10.1 > there is no error :)