Add a -l flag to Swift and use it to provide autolinking information.

The spelling of the flag can certainly be changed; I just wanted to get
something up and running.

Swift SVN r7582

lib/Frontend/CompilerInvocation.cpp

@@ -123,6 +123,10 @@ bool CompilerInvocation::parseArgs(ArrayRef<const char *> Args,
     case OPT_enable_definite_init:
       LangOpts.UseDefiniteInit = true;
       break;
+
+    case OPT_link_library:
+      addLinkLibrary(InputArg->getValue(), LibraryKind::Library);
+      break;
     }
   }