[openmp][mlir] Lower parallel if to new fork_call_if function.

This patch adds a new runtime function `fork_call_if` and uses that
to lower parallel if statements when going through OpenMPIRBuilder.

This fixes an issue where the OpenMPIRBuilder passes all arguments to
fork_call as a struct but this struct is not filled corretly in the
non-if branch by handling the fork inside the runtime.

Differential Revision: https://reviews.llvm.org/D138495