Ignore implicity casts for zero-as-null-pointer-constant warning

The repro in https://bugs.llvm.org/show_bug.cgi?id=34362
caused the left nullptr to be cast to a int* implicitly, which
resulted diagnosing this falsely.

Differential Revision: https://reviews.llvm.org/D39301

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@316605 91177308-0d34-0410-b5e6-96231b3b80d8
(cherry picked from commit 1c6ec39fd744fa88b373ec68d31a5ddb43cf932f)
2 files changed