[OpenMP] Fix `omp_get_wtime` function being marked incorrectly as readonly

OpenMP has a list of of optimistic attributes that can be attached to
known runtime functions to aid some analysis. The `omp_get_wtime`
function incorrectly used the `readonly` attribute. This is not correct
at the `omp_get_wtime` function changes values depending on some
external state. This is more correctly modeled with
`inaccessiblememonly` meaning that the value does not depend on anything
within the module, but can not be removes as it depends on external
state.

Fixes #57578

Reviewed By: tianshilei1992

Differential Revision: https://reviews.llvm.org/D133360