Remove -l flag from Swift interim driver.

Being able to pass -l to the driver isn't so interesting, and it's an
extra field that lives on TranslationUnit for no reason. Just remove it.

This doesn't interfere with autolinking, i.e. inferring -l flags based on
imported modules.

Swift SVN r9241

lib/Frontend/Frontend.cpp

@@ -123,8 +123,6 @@ void swift::CompilerInstance::doIt() {
   TU = new (*Context) TranslationUnit(ID, *Context);
   Context->LoadedModules[ID.str()] = TU;
 
-  TU->setLinkLibraries(Invocation.getLinkLibraries());
-
   auto *SingleInputFile =
     new (*Context) SourceFile(*TU, Kind, Invocation.getParseStdlib());
   TU->MainSourceFile = SingleInputFile;