[DSLX] Not able to assert on parametrics (as constexpr) in `const_assert!`

**Describe the bug**

Parametric values are flagging an issue in `const_assert!` as not being registered as constexpr -- I think this happens when we provide an incomplete set of parametrics -- the phantom parametric probably resolves to a `ParametricSymbol` (which we have intent to rip out) but the body of the function is still being evaluated for constexprs somehow. cc @richmckeever

**To Reproduce**

```
$ ~/opt/xlsynth/latest/dslx_interpreter_main reverse_chunks.x
reverse_chunks.x:19:3-19:36
0017:
0018: fn reverse_chunks(x: bits[FLAT]) -> bits[FLAT] {
0019: const_assert!(FLAT % N == u32:0); // Ensure FLAT is a multiple of N.
~~~~~~~~^-------------------------------^ TypeInferenceError: const_assert! expression is not constexpr
0020: const NUM_CHUNKS = FLAT / N;
```

Minimal reproducer:

```
fn f(x: bits[FLAT]) {
const_assert!(FLAT % N == u32:0);
}

#[test]
fn test_f() {
f(bits[42]:0)
}
```

```
$ ~/opt/xlsynth/latest/dslx_interpreter_main reverse_chunks.x
reverse_chunks.x:2:3-2:36
0001: fn f(x: bits[FLAT]) {
0002: const_assert!(FLAT % N == u32:0);
~~~~~~~~^-------------------------------^ TypeInferenceError: const_assert! expression is not constexpr
0003: }
0004:
```

**Expected behavior**
I'd expect this expression to be constexpr as the parametric values are compile-time constant, or to flag that not enough parametrics were provided to instantiate the function.