[mlir][Transforms] Make `ConversionPatternRewriter` constructor private (#82244)

`ConversionPatternRewriter` objects should not be constructed outside of
dialect conversions. Some IR modifications performed through a
`ConversionPatternRewriter` are reflected in the IR in a delayed fashion
(e.g., only when the dialect conversion is guaranteed to succeed). Using
a `ConversionPatternRewriter` outside of the dialect conversion is
incorrect API usage and can bring the IR in an inconsistent state.

Migration guide: Use `IRRewriter` instead of
`ConversionPatternRewriter`.