MemoryRegion: Print "don't know" permission values as such

Summary:
The permissions in a memory region have ternary states (yes, no, don't
know), but the memory region command only prints in binary, treating
"don't know" as "yes", which is particularly confusing as for instance
the unwinder will treat an unknown value as "no".

This patch makes is so that we distinguish all three states when
printing the values, using "?" to indicate the lack of information. It
is implemented via a special argument to the format provider for the
OptionalBool enumeration.

Reviewers: clayborg, jingham

Subscribers: lldb-commits

Differential Revision: https://reviews.llvm.org/D69106