[OpenMP] Fix mangling for linear modifiers with variable stride

This adds support for variable stride with the val, uval, and ref linear
modifiers.  Previously only the no modifer type ls<argno> was supported.

  val  -> Ls<argno>
  uval -> Us<argno>
  ref  -> Rs<argno>

Differential Revision: https://reviews.llvm.org/D125330