[funcattrs] Infer access attributes for vararg arguments

This change allows us to infer access attributes (readnone, readonly) on arguments passed to vararg functions. Since there isn't a formal argument corresponding to the parameter, they'll never be considered part of the speculative SCC, but they can still benefit from attributes on the call site or the callee function.

The main motivation here is just to simplify the code, and remove some special casing. Previously, an indirect vararg call could return more precise results than an direct vararg call which is just weird.

Differential Revision: https://reviews.llvm.org/D115964