[C++11] Use 'nullptr'.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@209612 91177308-0d34-0410-b5e6-96231b3b80d8
[C++11] Use 'nullptr'. Sema edition.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@209613 91177308-0d34-0410-b5e6-96231b3b80d8