Re-add -l flag to Swift interim driver.

This is useful in -i and REPL modes, which do not have a separate linking
step. This version of the -l flag doesn't rely on TranslationUnit. I also
cleaned up the library-loading diagnostics.

Swift SVN r9488

lib/Frontend/CompilerInvocation.cpp

@@ -127,6 +127,10 @@ bool CompilerInvocation::parseArgs(ArrayRef<const char *> Args,
       LangOpts.DebugConstraintSolver = true;
       break;
 
+    case OPT_link_library:
+      addLinkLibrary(InputArg->getValue(), LibraryKind::Library);
+      break;
+      
     case OPT_std_EQ:
       if (strcmp(InputArg->getValue(), "axle") == 0)
         LangOpts.Axle = true;