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