-Put TR_RegisterCandidate, TR_RegisterCandidates, and GlobalSet classes
into the OMR namespace and create derived classes in TR namespace,
allowing downstream projects to add a project-level class to those
hierarchies in order to extend functionality.
-Make members of the above classes protected
-Make the following methods virtual:
  -GlobalSet::collectReferencedAutoSymRefs()
  -RegisterCandidate::symbolIsLive()
  -RegisterCandidate::processLiveOnEntryBlocks()
  -RegisterCandidates::findOrCreate()
  -RegisterCandidates::find(TR::SymbolReference *)
  -RegisterCandidates::assign()
  -RegisterCandidates::computeAvailableRegisters()

-replace uses of TR_GlobalRegister with TR::GlobalRegister

-the following GRA methods are made virtual for downstream projects
to be able to extend functionality:
	-markAutosUsedIn()
	-transformNode()
	-prepareForBlockExit()
	-registerIsLiveAcrossEdge()
-make several GRA members protected
-make TR_LiveRangeSplitter members protected

Closes: eclipse#6529