Portability fix: add missing includes and static_casts. Reviewed as...

Portability fix: add missing includes and static_casts. Reviewed as https://reviews.llvm.org/D55777. Thanks to Andrey Maksimov for the patch.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@349566 91177308-0d34-0410-b5e6-96231b3b80d8