Don't include outer-most explicit cast in nullptr replacement

The outer-most explicit cast is now left alone by the Use-Nullptr transform to
maintain the type of the expression and avoid introducing ambiguities.

Fixes PR15395.

Author: Ariel J Bernal <ariel.j.bernal@intel.com>


git-svn-id: https://llvm.org/svn/llvm-project/clang-tools-extra/trunk@177179 91177308-0d34-0410-b5e6-96231b3b80d8