Re-re-apply 5acd4716, Add a shared-memory based orc::MemoryMapper...

...with more fixes.

The original patch was reverted in 3e9cc543 due to bot failures caused by
a missing dependence on librt. That issue was fixed in 32d8d23c, but that
commit also broke sanitizer bots due to a bug in SimplePackedSerialization:
empty ArrayRef<char>s triggered a zero-byte memcpy from a null source. The
ArrayRef<char> serialization issue was fixed in 67220c2a, and this patch has
also been updated with a new custom SharedMemorySegFinalizeRequest message that
should avoid serializing empty ArrayRefs in the first place.

https://reviews.llvm.org/D128544