Precise parsing of ` dense<0> : tensor<4xi8>`.

`AttrParser.lean`'s `parseOptionalDenseElementsAttr` parses the value and the type using `parseUnregisteredAttrBody` which returns a `String`. Make this more precise by defining a proper type for dense elements, and parsing into that.
Fixing this should result in a better parse tree for ` Test/LLVM/parse_zeroed_array_initializers.mlir`.

Start in AttrParser.lean at parseOptionalDenseElementsAttr, where the dense elements value and type are parsed with parseUnregisteredAttrBody. Define the proper dense-elements type and parse into it, then check Test/LLVM/parse_zeroed_array_initializers.mlir to confirm the parse tree is more precise.

Written by the indexing model from the issue text.