Fixed Use-Nullptr when replacing return(0)

Before fix, the paren expression was being replaced resulting in returnnullptr.
ParenExpr and implicit casts now ignored so we get return(nullptr) instead.

Added new test cases.

Fixes PR15398

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


git-svn-id: https://llvm.org/svn/llvm-project/clang-tools-extra/trunk@176551 91177308-0d34-0410-b5e6-96231b3b80d8