Internal type parameter representation in error

```fsharp
let (Some x) = None
x <- 123

```

It fails to type check the right side expression and fails with this error:

The only scenario I've seen it on is when it's inside a set expression and:
* the left side value is immutable
* the left side value appears to not yet inferred to have any constraints

This started showing up after #7682, but before that PR it would simply fail just before and wouldn't try to type check the right side expression at all.