[mlir][LLVM][NFC] Move `LLVMStructType` to ODS (#117485)

This PR extracts NFC changes out of
https://github.com/llvm/llvm-project/pull/116035 to reap as many of the
same benefits without any of the semantic changes.

More concretely, moving `LLVMStructType` to ODS has the benefits of
being able to generate much of the required boilerplate, such as
interface definitions, documentation and more, automatically.
Furthermore, `LLVMStructType` is then treated less special and its
definition can be found at the same place where all other complex type
definitions are found in the LLVM dialect.

Future changes could leverage more automatically generated code from
TableGen such as `assemblyFormat`. As these are not as trivial, they
have been left for future PRs.

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

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