Enable -Wcpp-undef in Hadrian

In !6340 (closed), @doyougnu reports:

The panics were caused by #ifdef UNIQUE_TAG_BITS == 8 evaluating to the else branch even on 64-bit machines. Which was due to the missing #include Unique.h which defines UNIQUE_TAG_BITS.

It should have been easier to spot. We should pass -Wcpp-undef in Hadrian.

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