Parametric values have invalid manglings into IR symbols

Previously the parametric values were positive integral, but now we're supporting more arbitrary `InterpValue`s, so we need to either define mangling policies for InterpValues into valid IR symbols or allow collisions to be avoided in IR symbols via attributes.

Sample that errors:

`fn __sample__x4__-76582139(x5: (bits[22])) -> (bits[1], bits[1], bits[28]) {`