Fix some warnings introduced in r353324 (ReproducerInstrumentation patch)

GetIndexForObjectImpl generated a bunch of "conversion casts away
constness warnings". Change the function to use "const void *" (and
static_cast, while I'm at it), to avoid this.

Driver.cpp: unused variable "replay" (this was actually caused by a
subsequent partial revert of this patch). I just finish the revert by
removing the variable completely.

llvm-svn: 353405