FileManager: Use llvm::Expected in new getFileRef API

`FileManager::getFileRef` is a modern API which we expect to convert to
over time.  We should modernize the error handling as well, using
`llvm::Expected` instead of `llvm::ErrorOr`, to help clients that care
about errors to ensure nothing is missed.

However, not all clients care.  I've also added another path for those
that don't:

- `FileEntryRef` is now copy- and move-assignable (using a pointer
  instead of a reference).
- `FileManager::getOptionalFileRef` returns an `llvm::Optional` instead
  of `llvm::Expected`.
- Added an `llvm::expectedToOptional` utility in case this is useful
  elsewhere.

https://reviews.llvm.org/D66705

llvm-svn: 369943