[mlir][Transforms] Dialect conversion: Add option to disable folding

This commit adds a new flag to `ConversionConfig` that deactivates op folding during a dialect conversion.

Op folding is problematic beause op folders may assume that the IR is in a valid state. (See #89770 for an example.) However, the dialect conversion driver does not guarantee that the IR is valid during a dialect conversion; it only guarantees that the IR is valid at the end of a dialect conversion. E.g., IR may be invalid after a conversion pattern application because some IR modifications (e.g., op/block replacements) are applied in a delayed fashion at the end of a dialect conversion. This makes op folders generally unsafe to use with a dialect conversion.

Note: For the same reason, it is also not safe to use non-conversion patterns with a dialect conversion. Conversion patterns can be used safely because they have an "adapter". (And conversion patterns cannot assume that the entire IR is valid in general.)