Fix specialization of SomeType<N> when N is a const parameter.

[jorendorff]

See #761. The test case that triggers the bug is like this:

#[repr(C)]
pub struct Inner<const N: usize> {
    pub bytes: [u8; N],
}

#[repr(C)]
pub struct Outer<const N: usize> {
    pub inner: Inner<N>, // BUG: declares two different structs named `Inner_N`
}

...code that uses Outer<1> and Outer<2>...

When monomorphizing Outer<1>, specializing the type Inner<N> should result in Inner<1>, but it produced Inner<N> instead. This happens because N was initially parsed as a type, not a constant, so we were thoughtlessly calling Type::specialize on it. This can't possibly work; the desired answer 1 is not a type.

The fix is to explicitly handle this special case where the identifier was initially parsed as a type but it's a const.

[jorendorff]

Friendly review ping. :)