[flang][OpenMP] Add semantic check to allow only loop iteration variables in a...

[flang][OpenMP] Add semantic check to allow only loop iteration variables in a `linear` clause within `distribute` construct.

Implement the following semantic check:

"A list item may not appear in a linear clause, unless it is the loop iteration variable."

Reviewed By: kiranchandramohan

Differential Revision: https://reviews.llvm.org/D100224