Auto merge of rust-lang#3385 - Zoxc:read-types, r=RalfJung

Report retags as distinct from real memory accesses for data races

This changes the error reporting for data races such that reference invariants are no longer reported as real read and writes.

Before:
```
Data race detected between (1) non-atomic write on thread `unnamed-6` and (2) non-atomic read on thread `unnamed-5` at alloc1034971+0x10c. (2) just happened here
```

After:
```
Data race detected between (1) non-atomic write on thread `unnamed-8` and (2) shared reference invariant on thread `unnamed-6` at alloc1018329+0x190. (2) just happened here
```

Non-atomic read accesses from the *other* thread don't have this information tracked so those are called `some potential non-atomic read access` here.

Author: bors