Fix bug in modernize-use-nullptr.

Summary:
https://llvm.org/bugs/show_bug.cgi?id=24960

modernize-use-nullptr would hit an assertion in some cases involving macros and initializer lists, due to finding a node with more than one parent (the two forms of the initializer list).

However, this doesn't mean that the replacement is incorrect, so instead of just rejecting this case I tried to find a way to make it work. Looking at the semantic form of the InitListExpr made sense to me (looking at both forms results in false negatives) but I am not sure of the things that we can miss by skipping the syntactic form.

Reviewers: klimek

Subscribers: cfe-commits, alexfh

Differential Revision: http://reviews.llvm.org/D13246

llvm-svn: 249291