declared char constants emit higher bit-width MLIR constants about polygeist

[j2kun]

TIL this is part of the C standard: all arithmetic is done at int level or higher.

[wsmoses]

if helpful, being the compiler we can make a type or option which actually enforces this

[j2kun]

@wsmoses It would be helpful for my project, but I'm not sure what's the right way to approach it, because I'm far from an expert in these sort of minute details of the C standard. You think it would make sense to add an option to cgeist to remove these implicit casts inserted in the Clang AST? Or would it make more sense to post-processes the MLIR and match on extsi -> binop -> trunci to simplify it?