[OpenMP][NFC] Consolidate `to` and `from` clause modifiers

`to` and `from` clauses take the same modifiers, which are called
"motion modifiers" in TR8, so implement handling of their modifiers
once not twice.  This will make it easier to implement additional
motion modifiers in the future.

Reviewed By: ABataev

Differential Revision: https://reviews.llvm.org/D84710