[OpenMP] Introduce "loop sequence" as directive association (#168934)
OpenMP 6.0 introduced a `fuse` directive, and with it a "loop sequence"
as the associated code. What used to be "loop association" has become
"loop-nest association".
Rename Association::Loop to LoopNest, add Association::LoopSeq to
represent the "loop sequence" association.
Change the association of fuse from "block" to "loop sequence".