[Patch] Driver: Warn on -mcmode=kernel on Darwin

[Vedant Kumar]

The 'kernel' mcmodel is not fully supported on Darwin (PR 19131741).

This patch causes the driver to spit out a warning when users pass -mcmodel=kernel in.

I was asked to add this warning because the alternatives are (1) crashing mysteriously in SelectionDAG, or (2) erroring out in the driver.

Since I don't have commit rights yet, I'd appreciate someone looking over this patch and guiding it into trunk.

thanks!
vedant

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