Fix broken C++ mode comment

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@221327 91177308-0d34-0410-b5e6-96231b3b80d8
IR: Metadata: Remove unnecessary dyn_cast

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@221328 91177308-0d34-0410-b5e6-96231b3b80d8