don't skip inference for type in offset_of!

tests/ui/offset-of/offset-of-inference.rs

@@ -0,0 +1,11 @@
+// Test that inference types in `offset_of!` don't ICE.
+
+#![feature(offset_of)]
+
+struct Foo<T> {
+    x: T,
+}
+
+fn main() {
+    let _ = core::mem::offset_of!(Foo<_>, x); //~ ERROR: type annotations needed
+}