[Clang][OpenMP][LoopTransformations] Implement "#pragma omp fuse" loop...

[Clang][OpenMP][LoopTransformations] Implement "#pragma omp fuse" loop transformation directive and "looprange" clause (#139293)

This change implements the fuse directive, `#pragma omp fuse`, as specified in the OpenMP 6.0, along with the `looprange` clause in clang.

This change also adds minimal stubs so flang keeps compiling (a full implementation in flang of this directive is still pending).

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Co-authored-by: Roger Ferrer Ibanez <roger.ferrer@bsc.es>