[ORC] Add new symbol lookup methods to ExecutionSessionBase in preparation for

deprecating SymbolResolver and AsynchronousSymbolQuery.

Both lookup overloads take a VSO search order to perform the lookup. The first
overload is non-blocking and takes OnResolved and OnReady callbacks. The second
is blocking, takes a boolean flag to indicate whether to wait until all symbols
are ready, and returns a SymbolMap. Both overloads take a RegisterDependencies
function to register symbol dependencies (if any) on the query.

llvm-svn: 337595