[ORC] Rewrite the VSO symbol table yet again. Update related utilities.

VSOs now track dependencies for materializing symbols. Each symbol must have its
dependencies registered with the VSO prior to finalization. Usually this will
involve registering the dependencies returned in
AsynchronousSymbolQuery::ResolutionResults for queries made while linking the
symbols being materialized.

Queries against symbols are notified that a symbol is ready once it and all of
its transitive dependencies are finalized, allowing compilation work to be
broken up and moved between threads without queries returning until their
symbols fully safe to access / execute.

Related utilities (VSO, MaterializationUnit, MaterializationResponsibility) are
updated to support dependence tracking and more explicitly track responsibility
for symbols from the point of definition until they are finalized.

llvm-svn: 332541

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