[flang][OpenMP][NFC] Share SetSymbolDSA between semantics and lowering (#207826)
Move the DSA helper from the private OmpAttributeVisitor::SetSymbolDSA
into the public openmp-dsa header next to GetSymbolDSA, and share the
DSA flag set through a single GetDataSharingAttributeFlags().
This lets an upcoming metadirective lowering change reuse the helper to
set the predetermined DSA of the loop induction variables of a selected
variant. A loop-associated variant is resolved during lowering, so the
usual semantic DSA resolution never runs on its loop nest and lowering
must set those flags itself.
Assisted with Copilot.