[lldb][NFCI] Change parameter type in Process::SetExitStatus

My primary motivation here is actually to change something in
UnixSignals, but this change is a necesary precondition.

I've also updated the documentation and rewritten the log statements to
use `formatv` instead of `printf` (printf-style formatting and
llvm::StringRef don't mix well).

Differential Revision: https://reviews.llvm.org/D157662