[OpenMP][Fix] Properly inherit calling convention

Previously in OpenMPOpt we did not correctly inherit the calling
convention of the callee when creating new OpenMP runtime calls. This
created issues when the calling convention was changed during
`GlobalOpt` but a new call was creating without the correct calling
convention. This lead to the call being replaced with a poison value in
`InstCombine` due to undefined behaviour and causing large portions of
the program to be incorrectly eliminated. This patch correctly inherits
the existing calling convention from the callee.

Reviewed By: tianshilei1992, jdoerfert

Differential Revision: https://reviews.llvm.org/D118059