[OpenMP] Refactor const restriction for reductions

[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

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@350440 91177308-0d34-0410-b5e6-96231b3b80d8