This change makes:
* #file compatible with #fileID in “Swift 6 mode”
* #file compatible with #filePath and #fileID in Swift 5 mode
* #file in Swift 5 mode code compatible with #file in “Swift 6 mode” code
This should keep anyone from seeing XCTAssert-wrapping noise until they adopt “Swift 6 mode” (whatever version that ends up actually being).