Prevents a user-visible warning on every compilation:

  ld: warning: directory not found for option '-L.../MacOSX11.0.sdk/usr/lib/swift'

--- a/lib/Driver/ToolChains.cpp
+++ b/lib/Driver/ToolChains.cpp
@@ -1455,9 +1455,11 @@ void ToolChain::getRuntimeLibraryPaths(SmallVectorImpl<std::string> &runtimeLibP
       runtimeLibPaths.push_back(std::string(scratchPath.str()));
     }
 
+    if (!SDKPath.startswith("@storeDir@")) {
     scratchPath = SDKPath;
     llvm::sys::path::append(scratchPath, "usr", "lib", "swift");
     runtimeLibPaths.push_back(std::string(scratchPath.str()));
+    }
   }
 }
 
--- a/lib/Frontend/CompilerInvocation.cpp
+++ b/lib/Frontend/CompilerInvocation.cpp
@@ -185,7 +185,9 @@ static void updateRuntimeLibraryPaths(SearchPathOptions &SearchPathOpts,
       RuntimeLibraryImportPaths.push_back(std::string(LibPath.str()));
     }
 
-    LibPath = SearchPathOpts.getSDKPath();
+    auto SDKPath = SearchPathOpts.getSDKPath();
+    if (!SDKPath.startswith("@storeDir@")) {
+    LibPath = SDKPath;
     llvm::sys::path::append(LibPath, "usr", "lib", "swift");
     if (!Triple.isOSDarwin()) {
       // Use the non-architecture suffixed form with directory-layout
@@ -200,6 +202,7 @@ static void updateRuntimeLibraryPaths(SearchPathOptions &SearchPathOpts,
       llvm::sys::path::append(LibPath, swift::getMajorArchitectureName(Triple));
     }
     RuntimeLibraryImportPaths.push_back(std::string(LibPath.str()));
+    }
   }
   SearchPathOpts.setRuntimeLibraryImportPaths(RuntimeLibraryImportPaths);
 }