[ORC] Simplify VSO::lookupFlags to return the flags map.

This discards the unresolved symbols set and returns the flags map directly
(rather than mutating it via the first argument).

The unresolved symbols result made it easy to chain lookupFlags calls, but such
chaining should be rare to non-existant (especially now that symbol resolvers
are being deprecated) so the simpler method signature is preferable.

llvm-svn: 337594