astgen: fill result location with void value if no other value

With this change, `break` and `break :blk` will fill the result location
with `.void_value`, ensuring that the value will be type checked.
The same will happen for a for loop that contains no `break`s in it's body.

Closes https://github.com/ziglang/zig/issues/14686.

test/cases/compile_errors/break_void_result_location.zig

@@ -0,0 +1,32 @@
+export fn f1() void {
+    const x: usize = for ("hello") |_| {};
+    _ = x;
+}
+export fn f2() void {
+    const x: usize = for ("hello") |_| {
+        break;
+    };
+    _ = x;
+}
+export fn f3() void {
+    var t: bool = true;
+    const x: usize = while (t) {
+        break;
+    };
+    _ = x;
+}
+export fn f4() void {
+    const x: usize = blk: {
+        break :blk;
+    };
+    _ = x;
+}
+
+// error
+// backend=stage2
+// target=native
+//
+// :2:22: error: expected type 'usize', found 'void'
+// :7:9: error: expected type 'usize', found 'void'
+// :14:9: error: expected type 'usize', found 'void'
+// :18:1: error: expected type 'usize', found 'void'