[DRR] Explicit Return Types in Rewrites

Adds a new rewrite directive returnType that can be added at the end of an op's
argument list to explicitly specify return types.

```
(OpX $v0, $v1, (returnType "$_builder.getI32Type()"))
```

Pass in a bound value to copy its return type, or pass a native code call to
dynamically create new types.

```
(OpX $v0, $v1, (returnType $v0, (NativeCodeCall<"..."> $v1)))
```

Reviewed By: jpienaar

Differential Revision: https://reviews.llvm.org/D109472