Driver: functionally revert the rt search change

Post-commit discussion with Jordan yielded another approach which is
viable and allows for cascading searches.  Revert the initial change
opting for the cascading search.

Author: compnerd

lib/Driver/ToolChains.cpp

@@ -1172,11 +1172,6 @@ void ToolChain::getResourceDirPath(SmallVectorImpl<char> &resourceDirPath,
   if (const Arg *A = args.getLastArg(options::OPT_resource_dir)) {
     StringRef value = A->getValue();
     resourceDirPath.append(value.begin(), value.end());
-  } else if (!getTriple().isOSDarwin() && args.hasArg(options::OPT_sdk)) {
-    StringRef value = args.getLastArg(options::OPT_sdk)->getValue();
-    resourceDirPath.append(value.begin(), value.end());
-    llvm::sys::path::append(resourceDirPath, "usr", "lib",
-                            shared ? "swift" : "swift_static");
   } else {
     auto programPath = getDriver().getSwiftProgramPath();
     resourceDirPath.append(programPath.begin(), programPath.end());