Collection expressions need to work with nullable-value-target-types

[CyrusNajmabadi]

Assigning to chuck pre-emptively so we don't miss this. If this already works (and has tests), great. If it doesn't, we need to make it work pre-launch:

ImmutableArray<int> a = [1, 2, 3];
ImmutableArray<int>? b = [1, 2, 3];
ImmutableArray<int?> c = [1, 2, 3];
ImmutableArray<int?>? d = [1, 2, 3];

This very strongly falls out from our correspondence with tuples. Specifically, all of the following work as expected:

(int, int) a = (1, 2);
(int, int)? b = (1, 2);
(int?, int?) c = (1, 2);
(int?, int?)? d = (1, 2);

Relates to test plan #66418

Spec update: dotnet/csharplang#7513