[ORC] Refactor the various lookupFlags methods to return the flags map via the

first argument.

This makes lookupFlags more consistent with lookup (which takes the query as the
first argument) and composes better in practice, since lookups are usually
linearly chained: Each lookupFlags can populate the result map based on the
symbols not found in the previous lookup. (If the maps were returned rather than
passed by reference there would have to be a merge step at the end).

llvm-svn: 323398