Add support for 1:N type conversions to the `FuncToLLVM` lowering
patterns. This commit does not change the lowering of any types (such as
`MemRefType`). It just sets up the infrastructure, such that 1:N type
conversions can be used during `FuncToLLVM`.
Note: When the converted result types of a `func.func` have more than 1
type, then the results are wrapped in an `llvm.struct`. That's because
`llvm.func` does not support multiple result values. This "wrapping" was
already implemented for cases where the original `func.func` has
multiple results. With 1:N conversions, even a single result can now
expand to multiple converted results, triggering the same wrapping
mechanism.
The test cases are exercised with both the old and the new no-rollback
conversion driver.
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