[ORC] Add a fallback definition generator for VSOs.

If a VSO has a fallback definition generator attached it will be called during
lookup (and lookupFlags) for any unresolved symbols. The definition generator
can add new definitions to the VSO for any unresolved symbol. This allows VSOs
to generate new definitions on demand.

The immediate use case for this code is supporting VSOs that can import
definitions found via dlsym on demand.

llvm-svn: 334538