[MSVC Compat] Accept elided commas in macro function arguments

Summary:
This fixes PR25875.  When the trailing comma in a macro argument list is
elided, we need to treat it similarly to the case where a variadic macro
misses one actual argument.

Reviewers: rnk, rsmith

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D15670

llvm-svn: 258530