make sure raw ptrs only have to be valid as far as they are used

tests/run-pass/deref_partially_dangling_raw.rs

@@ -0,0 +1,9 @@
+// Deref a raw ptr to access a field of a large struct, where the field
+// is allocated but not the entire struct is.
+// For now, we want to allow this.
+
+fn main() {
+    let x = (1, 1);
+    let xptr = &x as *const _ as *const (i32, i32, i32);
+    let _val = unsafe { (*xptr).1 };
+}