[Pitch] Add a filePath property to SourceLocation in Swift Testing

[jerryjrchen (Jerry Chen)]

This is helpful for the interop work in [Pitch] Targeted Interoperability between Swift Testing and XCTest. When corelibs XCTest reports an issue, it includes the source file path in the message using #file. To accurately translate Swift Testing issues to XCTest, we'd need this filePath too.