Instead, when a MaterializationResponsibility contains an initializer symbol,
the Platform classes (MachO, COFF, ELFNix) will now add a defined symbol with
the same name to an arbitary block within the initializer sections, and then
add keep-alive edges from that symbol to all other init section blocks.
ObjectLinkingLayer is updated to automatically discard symbols where the
corresponding MaterializationResponsibility entry has the
MaterializationSideEffecstsOnly flag. This change simplifies both the
ObjectLinkingLayer::Plugin interface and the dependence tracking algorithm,
which no longer needs a special case for "synthetic"
(MaterializationSideEffectsOnly) symbols.
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