fix handling of comptime-only union fields in Type.getUnionLayout (#25182)

Fixes #25180

test/behavior/union.zig

@@ -2321,3 +2321,21 @@ test "initialize empty field of union inside comptime-known struct constant" {
     const val: Wrapper = .{ .inner = .{ .none = {} } };
     comptime assert(val.inner.none == {});
 }
+
+test "union with function body field" {
+    const U = union {
+        f: fn () void,
+        fn foo() void {}
+        fn bar() void {}
+    };
+    const x: U = .{ .f = U.foo };
+    try std.testing.expect(x.f == U.foo);
+    x.f();
+
+    comptime var y: U = .{ .f = U.bar };
+    try std.testing.expect(y.f == U.bar);
+    y.f();
+    y.f = U.foo;
+    try std.testing.expect(y.f == U.foo);
+    y.f();
+}