[mlir] Allow IntegerAttr to parse zero width integers.

https://reviews.llvm.org/D109555 added support to APInt for this, so the special case to disable it is no longer valid. It is in fact legal to construct these programmatically today, and they print properly but do not parse.

Justification: zero bit integers arise naturally in various bit reduction optimization problems, and having them defined for MLIR reduces special casing.

I think there is a solid case for i0 and ui0 being supported. I'm less convinced about si0 and opted to just allow the parser to round-trip values that already verify. The counter argument is that the proper singular value for an si0 is -1. But the counter to this counter is that the sign bit is N-1, which does not exist for si0 and it is not unreasonable to consider this non-existent bit to be 0. Various sources consider it having the singular value "0" to be the least surprising.

Reviewed By: lattner

Differential Revision: https://reviews.llvm.org/D116413