[PATCH] Declare that musttail calls in variadic functions forward the ellipsis

[Reid Kleckner]

>>! In D4892#9, @nlewycky wrote:
> I haven't audited the codebase looking for places that assume that variadic callees must be passing in a discrete list of args, but every place I can think of (DAE, argpromotion, ipsccp) either skips varargs functions entirely or doesn't try to reason about the varargs part. Should be fine.

I see a problem in DAE, actually. I'll handle that in a separate commit. The test case for that feature is based on grep and needs work.

http://reviews.llvm.org/D4892