Default literals should not contribute their type from initial binding to best-type in nullable analysis

[RikkiGibson]

SharpLab

void M(bool b)
{
    var x = b ? default! : new[] {"1"};
    x[0] = null; // missing warning
}

The problem is that the BoundDefaultExpression representing default!, which was created by target typing a BoundDefaultLiteral, is contributing its type from initial binding string~[] to the best-type analysis of the conditional operator.

We could fix this, but I consider it lower priority than the collection-expr cases and some of the other cases that just fell out from my work in #74490.

I think the way to fix this would be to make a change to treat BoundDefaultExpression as a target-typed expression, adding a bool WasTargetTyped to it, and fitting it into the target-typed completion mechanism so that the NullableWalker's assumptions are satisfied.