[orc-rt] Add required/weak-ref attribute to NativeDylibManager lookup. (#201272)
Replace the std::vector<std::string> argument to lookup() with a
SymbolLookupSet (a vector of (name, LookupFlags) pairs, where
LookupFlags is RequiredSymbol or WeaklyReferencedSymbol).
This brings NativeDylibManager more closely into alignment with the
SimpleExecutorDylibManager implementation in the LLVM OrcTargetProcess
library.