Fix Clang-tidy modernize-use-nullptr and modernize-use-override warnings; other minor fixes.

Differential revision: reviews.llvm.org/D16568


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@258831 91177308-0d34-0410-b5e6-96231b3b80d8
21 files changed