Coding style fixup; no functionality change

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@204371 91177308-0d34-0410-b5e6-96231b3b80d8
Use nullptr; no functionality change

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@204372 91177308-0d34-0410-b5e6-96231b3b80d8