Undefined struct fields are initialized to 0 instead of 0xAA in debug mode (with llvm backend)

[IntegratedQuantum]

Zig Version

0.13.0-dev.75+5c9eb4081

Steps to Reproduce and Observed Behavior

const std = @import("std");

pub const Biome = struct {
	val: u32 = undefined,
};

pub fn main() !void {
	var biome1: Biome = .{};
	std.log.err("{any}", .{std.mem.asBytes(&biome1)});
	var biome2: Biome = .{.val = undefined};
	std.log.err("{any}", .{std.mem.asBytes(&biome2)});
	var biome3: Biome = undefined;
	std.log.err("{any}", .{std.mem.asBytes(&biome3)});
}

Note how only the last variant actually contains the desired 0xAA (170) bytes:

$ zig run test.zig
error: { 0, 0, 0, 0 }
error: { 0, 0, 0, 0 }
error: { 170, 170, 170, 170 }

Expected Behavior

Setting things to undefined should set them to 0xAA in Debug mode, like it does with the x86 backend.
This makes some bugs easier to identify.

Consider for example an ArrayListUnmanaged that gets initialized to undefined, like in #20085.
Since it gets currently set to 0 in debug mode, the deinit() does not produce any errors, making it only possible to identify the issue in release.
With 0xAA it would produce a runtime crash when attempting to free it and according to zig zen:

• Runtime crashes are better than bugs.

[FnControlOption]

Relevant: https://reviews.llvm.org/D58188

[vadim-za]

A similar situation happens with optionals. Could it be the same issue?

const std = @import("std");

pub fn main() void {
    var i: ?i32 = null;
    std.debug.print("{x}\n", .{std.mem.asBytes(&i)});
    i = undefined;
    std.debug.print("{x}\n", .{std.mem.asBytes(&i)});
    i = @as(i32, undefined);
    std.debug.print("{x}\n", .{std.mem.asBytes(&i)});
}

prints:

{ 0, 0, 0, 0, 0, 0, 0, 0 }
{ aa, aa, aa, aa, aa, aa, aa, aa }
{ 0, 0, 0, 0, 1, 0, 0, 0 }

instead of expected in the 3rd line:

{ aa, aa, aa, aa, 1, 0, 0, 0 }