[JITLink] Add JITLinkDylib type, thread through JITLinkMemoryManager APIs.

[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).