[mlir] Make `StringRefParameter` roundtrippable (#65813)

The current printer of `StringRefParameter` simply prints out the
content of the string as is without escaping it any way. This leads to
it generating invalid syntax, causing parser errors when read in again.

This PR fixes that by adding `printString` to `AsmPrinter`, allowing one
to print a string that can be parsed with `parseString`, using the same
escaping syntax as `StringAttr`.