[mlir][python] Support more types in IntegerAttr.value

Previously only accessing values for `index` and signless int types
would work; signed and unsigned ints would hit an assert in
`IntegerAttr::getInt`. This exposes `IntegerAttr::get{S,U}Int` to the C
API and calls the appropriate function from the python bindings.

Reviewed By: ftynse

Differential Revision: https://reviews.llvm.org/D120194