[OpenMP] Refactor const restriction for reductions

As discussed in D56113, this patch refactors the implementation of the
const restriction for reductions to reuse a function introduced by
D56113.  A side effect is that diagnostics sometimes now say
"variable" instead of "list item" when a list item is a variable.

Reviewed By: ABataev

Differential Revision: https://reviews.llvm.org/D56298

llvm-svn: 350440