Internalize Driver lto option

Author: kateinoigakukun

Sources/SwiftDriver/Driver/Driver.swift

@@ -166,7 +166,7 @@ public struct Driver {
   @_spi(Testing) public let compilerOutputType: FileType?
 
   /// The type of the link-time-optimization we expect to perform.
-  public let lto: LTOKind?
+  @_spi(Testing) public let lto: LTOKind?
 
   /// The type of the primary output generated by the linker.
   @_spi(Testing) public let linkerOutputType: LinkOutputType?