Fix UseNullptr fails to replace explict casts surrounded by another implicit
cast

UseNullptr previously matched the implicit cast to const pointer as well as
the explicit cast within that has an implicit cast to nullptr as a descendant.

-Refactored UseNullptr to avoid special-casing certain kinds of cast sequences
-Added test cases.


git-svn-id: https://llvm.org/svn/llvm-project/clang-tools-extra/trunk@178907 91177308-0d34-0410-b5e6-96231b3b80d8