Sema: resolve union fields before using getTagType

Closes #13649

test/cases/compile_errors/union_fields_are_resolved_before_tag_type_is_needed.zig

@@ -0,0 +1,16 @@
+const T = union(enum) {
+    a,
+    pub fn f(self: T) void {
+        _ = self;
+    }
+};
+pub export fn entry() void {
+    T.a.f();
+}
+
+// error
+// backend=stage2
+// target=native
+//
+// :8:8: error: no field or member function named 'f' in '@typeInfo(tmp.T).Union.tag_type.?'
+// :1:11: note: enum declared here