[OpenMP] Make OpenMPOpt aware of the OpenMP runtime's status

The `OpenMPOpt` pass contains optimizations that generate new calls into
the OpenMP runtime. This causes problems if we are in a state where the
runtime has already been linked statically. Generating these new calls
will result in them never being resolved. We should indicate if we are
in a "post-link" LTO phase and prevent OpenMPOpt from generating new
runtime calls.

Generally, it's not desireable for passes to maintain state about the
context in which they're called. But this is the only reasonable
solution to static linking when we have a pass that generates new
runtime calls.

Reviewed By: jdoerfert

Differential Revision: https://reviews.llvm.org/D142646