[OpenMP] Refactor const restriction for linear

As discussed in D56113, this patch refactors the implementation of the
const restriction for linear to reuse a function introduced by D56113.
A side effect is that, if a variable has mutable members, this
diagnostic is now skipped, and the diagnostic for the variable not
being an integer or pointer is reported instead.

Reviewed By: ABataev

Differential Revision: https://reviews.llvm.org/D56299

llvm-svn: 350441