Frontend: teach the compiler to use a backup directory to find .swiftinterface files to compile

This mechanism allows the compiler to use a backup interface file to build into a binary module when
a corresponding interface file from the SDK is failing for whatever reasons. This mechansim should be entirely opaque
to end users except several diagnostic messages communicating backup interfaces are used.

Part of rdar://77676064

lib/Serialization/ModuleDependencyScanner.cpp

@@ -54,6 +54,7 @@ std::error_code ModuleDependencyScanner::findModuleFilesInDirectory(
     }
   }
   assert(fs.exists(InPath));
+
   // Use the private interface file if exits.
   auto PrivateInPath =
   BaseName.getName(file_types::TY_PrivateSwiftModuleInterfaceFile);