[mlir] Rename `OpAsmParser::OperandType` to `OpAsmParser::UnresolvedOperand`

I am not sure about the meaning of Type in the name (was it meant be interpreted as Kind?), and given the importance and meaning of Type in the context of MLIR, its probably better to rename it. Given the comment in the source code, the suggestion in the GitHub issue and the final discussions in the review, this patch renames the OperandType to UnresolvedOperand.

Fixes https://github.com/llvm/llvm-project/issues/54446

Differential Revision: https://reviews.llvm.org/D122142