[DXIL] Don't generate per-variable guards for DirectX (#106096)
Thread init guards are generated for local static variables when using
the Microsoft CXX ABI. This ABI is also used for HLSL generation, but
DXIL doesn't need the corresponding _Init_thread_header/footer calls and
doesn't really have a way to handle them in its output targets.
This modifies the language ops when the target is DXIL to exclude this
so that they won't be generated and an alternate guardvar method is used
that is compatible with the usage.
Done to facilitate testing for #89806, but isn't really related