patch for warning on logical negation with known result

[jahanian]

I would like to warn on logical negation of operands with known non-null value; as in "if (!array)…", just as we warn on "if (array == 0)…"
Note that this is a c-only patch. c++ already has a warning when such non-null pointers are implicitly converted to bool. 
Is this ok for check-in?

- Fariborz

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