Swift: Fix quoting of library search paths with spaces

The library search paths added by commit 2746c61e6d (Swift: Add library
search paths for dependencies, 2019-06-09, v3.16.0-rc1~561^2) need to be
quoted properly on command lines to handle spaces and such.  This was
already done by `cmLinkLineComputer::ComputeLinkPath` for
non-Swift-specific link directories.

Source/cmLinkLineComputer.cxx

@@ -142,9 +142,11 @@ void cmLinkLineComputer::ComputeLinkPath(
           type = cmStateEnums::ImportLibraryArtifact;
         }
 
-        linkPathNoBT += cmStrCat(
-          " ", libPathFlag, item.Target->GetDirectory(cli.GetConfig(), type),
-          libPathTerminator, " ");
+        linkPathNoBT +=
+          cmStrCat(" ", libPathFlag,
+                   this->ConvertToOutputForExisting(
+                     item.Target->GetDirectory(cli.GetConfig(), type)),
+                   libPathTerminator, " ");
       }
     }