[JITLink] Add JITLinkDylib type, thread through JITLinkMemoryManager APIs.
JITLinkDylib represents a target dylib for a JITLink link. By representing this
explicitly we can:
- Enable JITLinkMemoryManagers to manage allocations on a per-dylib basis (e.g by maintaining a seperate allocation pool for each JITLinkDylib).
- Enable new features and diagnostics that require information about the target dylib (not implemented in this patch).