Fix build error caused by https://reviews.llvm.org/D149718

The patch(https://reviews.llvm.org/D149718) broke buildbot

../../clang/include/clang/Sema/ParsedAttr.h:705:18: error: explicitly defaulted move assignment operator is implicitly deleted [-Werror,-Wdefaulted-function-deleted]
  AttributePool &operator=(AttributePool &&pool) = default;
                 ^
../../clang/include/clang/Sema/ParsedAttr.h:674:21: note: move assignment operator of 'AttributePool' is implicitly deleted because field 'Factory' is of reference type 'clang::AttributeFactory &'
  AttributeFactory &Factory;
                    ^
1 error generated.

This patch fixes the build error.