[ORC] Make EPCDynamicLibrarySearchGenerator async

Switch the primary implementation of EPC lookupSymbols to be async,
keeping a synchronous wrapper for compatibility. Use the new async
implementation inside EPCDynamicLibrarySearchGenerator to work
working towards a fully async search generator.

Provide an asynchronous lookup API for EPCGenericDylibManager and adopt
that from the SimpleRemoteEPC. This enables an end-to-end async
EPCDynamicLibrarySearchGenerator. Note: currently we keep the current
per-dlhandle lookup model, but a future improvement could do a single
async call for a given lookup operation.