Add `const` to `dump` method of `OpFoldResult`.

While most `dump` methods are marked `const`, some arent marked as
`const`. Adding `const` to `OpFoldResult` here since this was
encountered as an issue while debugging (doing `dump` within a debug
console threw an error indicating the method should be marked
`const`).

Reviewed By: ftynse

Differential Revision: https://reviews.llvm.org/D135241