[ORC] Introduce ExecutorAddress type, fix broken LLDB bot.

ExecutorAddressRange depended on JITTargetAddress, but JITTargetAddress is
defined in ExecutionEngine, which OrcShared should not depend on.

This seems like as good a time as any to introduce a new ExecutorAddress type
to eventually replace JITTargetAddress. For now it's just another uint64_t
alias, but it will soon be changed to a class type to provide greater type
safety.