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

[Krzysztof Parzyszek via cfe-commits]

kparzysz wrote:

> Are you going to submit your code?

Yes, but it's still incomplete.  I can wait with this PR if you prefer.

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