[clang] [clang][OpenMP] Add const-qualified `getIteratorModifier` to OMPMapClause (PR #86666)

[Krzysztof Parzyszek via cfe-commits]

kparzysz wrote:

I don't think so, because nothing is failing without it.  This is just for completeness.

https://github.com/llvm/llvm-project/pull/86666