cpp11-migrate: Refactor NullptrActions

Several free functions related to macro arg testing are being moved into
CastSequenceVisitor to facilitate upcoming fix.



git-svn-id: https://llvm.org/svn/llvm-project/clang-tools-extra/trunk@182013 91177308-0d34-0410-b5e6-96231b3b80d8
cpp11-migrate: Check for valid NULL macros from macro arg expansions

The recent improvement to the Use Nullptr Transform for macro arg
expansions wasn't testing that only allowed NULL macros used in macro
args can be transformed. This revision replaces a TODO to that effect.

Fixes PR15955.



git-svn-id: https://llvm.org/svn/llvm-project/clang-tools-extra/trunk@182014 91177308-0d34-0410-b5e6-96231b3b80d8