[NFC][lldb] Implement DiagnosticManager::Consume (#74011)

In some situations it may be useful to have a separate DiagnosticManager
instance, and then later of move the contents of that instance back to
the "main" DiagnosticManager. For example, when silently retrying some
operation with different parameters, depending on whether the retry
succeeded or failed, LLDB may want to present a different set of
diagnostics to the user (the ones generated on the first try vs the
retry). Implement DiagnosticManager::Consume to allow for this use case.