[ORC] Fix symbol dependence propagation algorithm in ObjectLinkingLayer.

ObjectLinkingLayer was not correctly propagating dependencies through local
symbols within an object. This could cause symbol lookup to return before a
searched-for symbol is ready if the following conditions are met:
(1) The definition of the symbol being searched for transitively depends on a
    local symbol within the same object, and that local symbol in turn
    transitively depends on an external symbol provided by some other module
    in the JIT.
(2) Concurrent compilation is enabled.
(3) Thread scheduling causes the lookup of the searched-for symbol to return
    before all transitive dependencies of the looked-up symbol are emitted.

This bug was found by inspection and has not been observed in practice.

A jitlink test case has been added to verify that symbol dependencies are
correctly propagated through local symbol definitions.