[OPENMP]Fix PR50733: unexpected final value of list-item in linear clause in loop...

[OPENMP]Fix PR50733: unexpected final value of list-item in linear clause in loop construct.

Currently the last value of linear is calculated as var = init + num_iters * step.
Replaced it with var = var_priv, i.e. original variable gets the value
of the last private copy.

Differential Revision: https://reviews.llvm.org/D105151