[ORC] Replace lookupFlags in JITSymbolResolver with getResponsibilitySet.

The new method name/behavior more closely models the way it was being used.
It also fixes an assertion that can occur when using the new ORC Core APIs,
where flags alone don't necessarily provide enough context to decide whether
the caller is responsible for materializing a given symbol (which was always
the reason this API existed).

The default implementation of getResponsibilitySet uses lookupFlags to determine
responsibility as before, so existing JITSymbolResolvers should continue to
work.

llvm-svn: 340874