[MLIR] Translate DIStringType. (#94480)

This PR handle translation of DIStringType. Mostly mechanical changes to
translate DIStringType to/from DIStringTypeAttr. The 'stringLength'
field is 'DIVariable' in DIStringType. As there was no `DIVariableAttr`
previously, it has been added to ease the translation.

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Co-authored-by: Tobias Gysi <tobias.gysi@nextsilicon.com>