MGCA: Support tuple expressions as direct const arguments

[BoxyUwU]

We should support tuples expressions as direct const arguments so that tuple types as const generics work nicely.

#![feature(min_generic_const_args, adt_const_params)]

fn takes_tuple<const A: (u32, u32, u32)>() {}

fn generic_caller<const N: u32, const N2: u32>() {
    takes_tuple::<{ (N, N2, 1 + 1) }>();
}

These should lower directly to a ConstKind::Value in HIR ty lowering.

[mu001999]

@rustbot claim