[MLIR][DLTI] Pretty parsing and printing for DLTI attrs (#113365)

Unifies parsing and printing for DLTI attributes. Introduces a format of
`#dlti.attr<key1 = val1, ..., keyN = valN>` syntax for all queryable
DLTI attributes similar to that of the DictionaryAttr, while retaining
support for specifying key-value pairs with `#dlti.dl_entry` (whether to
retain this is TBD).

As the new format does away with most of the boilerplate, it is much easier
to parse for humans. This makes an especially big difference for nested
attributes.

Updates the DLTI-using tests and includes fixes for misc error checking/
error messages.

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