[mlir][sparse] Adding `{Var,DimLvlExpr,DimSpec,LvlSpec,DimLvlMap}::str` methods

These methods are needed for use with `Diagnostic::operator<<` etc.

The definitions follow the pattern of `Diagnostic::str` by simply wrapping the underlying `print(raw_ostream)` method.  Although there is some overhead for constructing the `std::string`, this seems like the overall most-efficient option: since this overhead only occurs on the error path (under the current intended usage).  An alternative approach would be to have one method construct a `Twine` directly, and then have the print method pass the twine to the stream; however, that would mean introducing the overhead of twine construction on the common/happy path of simply printing things out.

Reviewed By: aartbik

Differential Revision: https://reviews.llvm.org/D157643

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