Sema: handle foo() catch |err| switch (err) {...} for !noreturn

[JackoCoolio]

Fixes #25037. The issue there is that when analyzing a switch_block_err_union, we analyze the non-error case which includes an err_union_payload_unsafe instruction. zirErrUnionPayload sees that the OPV of the payload is unreachable, so it returns that without emitting any AIR. The assertion at the bottom of analyzeBodyInner then fails because block.instructions.items.len == 0:

if (sema.isNoReturn(air_inst)) {
    // We're going to assume that the body itself is noreturn, so let's ensure that now
    assert(block.instructions.items.len > 0);
    assert(sema.isNoReturn(block.instructions.items[block.instructions.items.len - 1].toRef()));
    break;
}

https://github.com/ziglang/zig/blob/0b75a2a1b1972777350c33c27cfe720172491de6/src/Sema.zig#L1979C9-L1984C10

---

My fix was just not analyzing the non-error case (by not bothering with the cond_br on is_non_err) when the payload type is noreturn.

Just skipping the assertions above when air_inst == .unreachable_value also seemed to work fine, but didn't feel great. Same for adding a sema.analyzeUnreachable(...) in zirErrUnionPayload.

I also replaced std.ArrayListUnmanaged with std.ArrayList throughout Sema.