[Interpreter] Clean up the /usr/lib/swift fallback code.

- Check the target triple at runtime to decide whether to use the fallback.
- Change isInResourceDir to actually check the resource dir.
- Use ArrayRef<std::string> instead of std::vector<std::string>.

lib/Frontend/ParseableInterfaceModuleLoader.cpp

@@ -1066,12 +1066,9 @@ class ParseableInterfaceModuleLoaderImpl {
   }
 
   bool isInResourceDir(StringRef path) {
-    for (auto &RuntimeLibraryPath : ctx.SearchPathOpts.RuntimeLibraryPaths) {
-      if (path.startswith(RuntimeLibraryPath)) {
-        return true;
-      }
-    }
-    return false;
+    StringRef resourceDir = ctx.SearchPathOpts.RuntimeResourcePath;
+    if (resourceDir.empty()) return false;
+    return path.startswith(resourceDir);
   }
 
   /// Finds the most appropriate .swiftmodule, whose dependencies are up to