[ADT/NullablePtr] Allow implicit conversion of NullablePtr<OtherT> -> NullablePtr<T> if OtherT is derived from T.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@185851 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed