[ORC] Lookup now returns an error if any symbols are not found.

[ORC] Lookup now returns an error if any symbols are not found.

Also tightens the behavior of ExecutionSession::failQuery. Queries can usually
only be failed by marking a symbol as failed-to-materialize, but
ExecutionSession::failQuery provides a second route, and both routes may be
executed from different threads. In the case that a query has already been
failed due to a materialization error, ExecutionSession::failQuery will
direct the error to ExecutionSession::reportError instead.

llvm-svn: 332898