Constexpr evaluation in impl methods doesn't always work

**Describe the bug**
`const_assert!()` doesn't seem to be able to constexpr-evaluate function parametrics.

**To Reproduce**
```
pub struct A {}
impl A {
fn B() -> bool {
const_assert!(C >= u32:4);
true
}
}
```

**Expected behavior**
It should typecheck and the assertion should pass or fail base on invocation.

Instead, I get `TypeInferenceError: const_assert! expression is not constexpr` regardless of the invocations of `A::B<...>()`.