Use bool in place of int for boolean things in the C API

`bool` is pretty well supported by now in C, and using it in place of `int` is not only more semantically accurate, but also improves automatic bindings for languages like Swift.

There is more discussion here: https://llvm.discourse.group/t/adding-mlirbool-to-c-bindings/2280/5

Reviewed By: ftynse, mehdi_amini

Differential Revision: https://reviews.llvm.org/D92193