[XLA:GPU][IndexAnalysis] Add DimVar, RangeVar and RTVar to IndexingMap.
[XLA:GPU][IndexAnalysis] Add DimVar, RangeVar and RTVar to IndexingMap.

RTVar is a new type of symbol/variable, that is associated with a runtime value of an HLO instruction, e.g. it can be used to model DUS, gather, etc.
