[Clang][OpenMP] Infix OMPLoopTransformationDirective abstract class. NFC.

Insert OMPLoopTransformationDirective between OMPLoopBasedDirective and the loop transformations OMPTileDirective and OMPUnrollDirective. This simplifies handling of loop transformations not requiring distinguishing between OMPTileDirective and OMPUnrollDirective anymore.

Reviewed By: ABataev

Differential Revision: https://reviews.llvm.org/D111119