[PATCH] Disallow clang from assigning a diagnostic to ID zero

[David Majnemer]

The fallout of r184401 made it apparent that clang uses diagnostic ID zero
as a sentinel value while *also* assigning it to an actual diagnostic,
err_attribute_not_type_attr.

The attached patch should do two things:
* Ensure that we don't give away ID 0
* Make sure that we will assert if we attempt to use ID 0

-- 
David Majnemer
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