[mlir][ODS] Use StringLiteral instead of StringRef when applicable

Use `StringLiteral` for function return type if it is known to return
constant string literals only.

This will make it visible to API users, that such values can be safely
stored, since they refers to constant data, which will never be deallocated.

`StringRef` is general is not safe to store for a long term,
since it might refer to temporal data allocated in heap.

Reviewed By: mehdi_amini, bkramer

Differential Revision: https://reviews.llvm.org/D95945