Promote null pointer constants used as arguments to variadic functions

Make it possible to pass NULL through variadic functions on 64-bit
Windows targets. The Visual C++ headers define NULL to 0, when they
should define it to 0LL on Win64 so that NULL is a pointer-sized
integer.

Fixes PR20949.

Reviewers: thakis, rsmith

Differential Revision: http://reviews.llvm.org/D5480

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@219456 91177308-0d34-0410-b5e6-96231b3b80d8