[ORC] Add helpers for building orc::SymbolResolvers from legacy findSymbol-style

functions/methods that return JITSymbols.

lookupFlagsWithLegacyFn takes a SymbolNameSet and a legacy lookup function and
returns a LookupFlagsResult. It uses the legacy lookup function to search for
each symbol. If found, getFlags is called on the symbol and the flags added to
the SymbolFlags map. If not found, the symbol is added to the SymbolsNotFound
set.

lookupWithLegacyFn takes an AsynchronousSymbolQuery, a SymbolNameSet and a
legacy lookup function. Each symbol in the SymbolNameSet is searched for via the
legacy lookup function. If it is found, its getAddress function is called
(triggering materialization if it has not happened already) and the resulting
mapping stored in the query. If it is not found the symbol is added to the
unresolved symbols set which is returned at the end of the function. If an
error occurs during legacy lookup or materialization it is passed to the
query via setFailed and the function returns immediately.

llvm-svn: 323388