Rollup merge of #118844 - celinval:smir-mono-args, r=compiler-errors

Monomorphize args while building Instance body in StableMIR

The function `Instance::body()` in StableMIR is supposed to return a monomorphic body by instantiating all possibly generic constructs. We were previously instantiating type and constants, but not generic arguments. This PR ensures that we also instantiate them.

r? ``@compiler-errors``

Author: matthiaskrgr