[llvm] [Frontend][OpenMP] Move isPrivatizingClause to OMP.h, NFC (PR #148644)

[Krzysztof Parzyszek via llvm-commits]

kparzysz wrote:

Stack:
- https://github.com/llvm/llvm-project/pull/148644 (this PR)
- https://github.com/llvm/llvm-project/pull/148647
- https://github.com/llvm/llvm-project/pull/148653
- https://github.com/llvm/llvm-project/pull/148654

https://github.com/llvm/llvm-project/pull/148644