[flang] [llvm] [flang][OpenMP] Gate the allocate clause at OpenMP 5.0 (PR #213980)

[Krzysztof Parzyszek via llvm-commits]

https://github.com/kparzysz approved this pull request.

Please trim the commit message to a simple description of the change.  One sentence is enough.

https://github.com/llvm/llvm-project/pull/213980