Sema: Fixing segfaults on mutually recursive structs

[antoniospg]

Trying to fix both #19920 and #18556
Not sure those are the best approaches.

4d580af Solves the following issue:

const A = struct {
    a: A,
};

pub fn main() !void {
    const a: A = undefined;
    _ = a;
}

On addDbgVar, it keeps caling hasRuntimeBitsAdvanced infinitely on the struct type.
I've tried to add resolveTypeFully to get the compiler error first.

7170203 Solves the following issue:

const A = struct {
    unused: void,
    bad_field: [@sizeOf(A)]u8 align(@alignOf(A)),
};

pub fn main() !void {
    const a: A = undefined;
    _ = a;
}

In this example, only the first field of the struct is non-null when calling haveFieldTypes which causes it to return true when it shouldn't.

[mlugg]

4d580af Solves the following issue:

This fix is not appropriate. We need to avoid infinite recursion on Type.hasRuntimeBitsAdvanced in the general case; for instance, I suspect your fix would fail if used in an -fstrip build. Fixing this correctly will probably involve some critical analysis of the whole "assumed runtime bits" system for structs, since it's a bit silly.

7170203 Solves the following issue:

This fix is also inappropriate, since it needlessly turns an O(1) operation into an O(N) operation. I recall fixing a bug similar to this one recently, actually... the easiest solution is probably to reset the first field type to .none if type resolution of any field fails.

Also, note that bugfixes such as these should add corresponding behavior tests whenever possible.