Sema: improve compile error for casting double pointer to anyopaque pointer

Closes #12042

test/cases/compile_errors/dont_implicit_cast_double_pointer_to_anyopaque.zig

@@ -1,14 +0,0 @@
-export fn entry() void {
-    var a: u32 = 1;
-    var ptr: *align(@alignOf(u32)) anyopaque = &a;
-    var b: *u32 = @ptrCast(*u32, ptr);
-    var ptr2: *anyopaque = &b;
-    _ = ptr2;
-}
-
-// error
-// backend=stage2
-// target=native
-//
-// :5:28: error: expected type '*anyopaque', found '**u32'
-// :5:28: note: pointer type child '*u32' cannot cast into pointer type child 'anyopaque'

test/cases/compile_errors/double_pointer_to_anyopaque_pointer.zig

@@ -0,0 +1,28 @@
+pub export fn entry1() void {
+    const x: usize = 5;
+
+    const ptr: *const anyopaque = &(&x);
+    _ = ptr;
+}
+pub export fn entry2() void {
+    var val: [*:0]u8 = undefined;
+    func(&val);
+}
+fn func(_: ?*anyopaque) void {}
+pub export fn entry3() void {
+    var x: *?*usize = undefined;
+
+    const ptr: *const anyopaque = x;
+    _ = ptr;
+}
+
+// error
+// backend=stage2
+// target=native
+//
+// :4:35: error: expected type '*const anyopaque', found '*const *const usize'
+// :4:35: note: cannot implicitly cast double pointer '*const *const usize' to anyopaque pointer '*const anyopaque'
+// :9:10: error: expected type '?*anyopaque', found '*[*:0]u8'
+// :9:10: note: cannot implicitly cast double pointer '*[*:0]u8' to anyopaque pointer '?*anyopaque'
+// :15:35: error: expected type '*const anyopaque', found '*?*usize'
+// :15:35: note: cannot implicitly cast double pointer '*?*usize' to anyopaque pointer '*const anyopaque'