Sema: improve circular dependency errors

test/cases/compile_errors/direct_struct_loop.zig

@@ -0,0 +1,9 @@
+const A = struct { a : A, };
+export fn entry() usize { return @sizeOf(A); }
+
+// error
+// backend=stage2
+// target=native
+//
+// :1:11: error: struct 'tmp.A' depends on itself
+// :1:20: note: while checking this field