__OwnedMutable

__OwnedMutable

Summary:
This diff introduces `__OwnedMutable` annotation that can be applied to only function parameters. When calling a function the only valid values of argument for parameter with `__OwnedMutable` are:
- `Rx\move(mutably-owned-local)`
- `Rx\mutable(new-or-call-expression)`

Also this diff introduces `Rx\move` intrinsic and changes the semantics of `Rx\freeze` to be aligned with `Rx\move`. In pseudocode:
```
namespace Rx;
<<__Rx, __MutableReturn>>
function move<T>(inout T $v): T {
   $tmp = $v; // grab the referenced value
   unset(v);   // unset local at the callsite
   return $tmp; // return mutably owned value
}
<<__Rx>>
function freeze<T>(inout T $v): T {
   $tmp = $v; // grab the referenced value
   unset(v);   // unset local at the callsite
   return $tmp; // return immutable value
}

```

Reviewed By: DavidSnider

Differential Revision: D10367849

fbshipit-source-id: 0280dc74c5d47d65341ba260ecd3693dbdeb0ba2