ast: Allow disambiguation of ConstGenericArgs to const generic arguments

At the AST level, we might encounter situations where const generic
arguments are ambiguous: They could be resolved to either a type or a
const expression, as shown in this example:

```rust
let a: Foo<N>; // what is N? A type? A const? The parser doesn't know
```

However, when parsing default expressions for const generic parameters,
we need to be able to disambiguate to const expressions early:

```rust
struct Foo<const N: usize = { M }> {
    /* ... */
}
```

In that code, `M` could be considered ambiguous: Is it a type? a const?
A random non-const variable? What matters is that we disambiguate it to
a const generic argument, so that it errors out appropriately in later
phases of the compiler.

In that case, we need to go from a `ConstArg::Ambiguous("M")` to a
`ConstArg::Clear(IdentifierExpr("M"))`.

In later passes of the compiler, we will also need to disambiguate to
types. But that will be done at the HIR/Resolving level.

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