Fix Clang-tidy modernize-use-nullptr warnings in source directories and generated files; other minor cleanups.

Patch by Eugene Zelenko!

Differential Revision: http://reviews.llvm.org/D13321

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@249482 91177308-0d34-0410-b5e6-96231b3b80d8
26 files changed