add UI regression test for offset_of! recovery

tests/ui/offset-of/offset-of-error-recovery.rs

@@ -0,0 +1,14 @@
+use std::mem::offset_of;
+
+struct S {
+    x: (),
+}
+
+impl S {
+    fn a() {
+        offset_of!(Self, Self::x);
+        //~^ ERROR offset_of expects dot-separated field and variant names
+    }
+}
+
+fn main() {}

tests/ui/offset-of/offset-of-error-recovery.stderr

@@ -0,0 +1,8 @@
+error: offset_of expects dot-separated field and variant names
+  --> $DIR/offset-of-error-recovery.rs:9:26
+   |
+LL |         offset_of!(Self, Self::x);
+   |                          ^^^^^^^
+
+error: aborting due to 1 previous error
+