Refactor DialectConversion to support different conversion modes.

Refactor DialectConversion to support different conversion modes.

Users generally want several different modes of conversion. This cl refactors DialectConversion to provide two:
* Partial (applyPartialConversion)
  - This mode allows for illegal operations to exist in the IR, and does not fail if an operation fails to be legalized.

* Full (applyFullConversion)
  - This mode fails if any operation is not properly legalized to the conversion target. This allows for ensuring that the IR after a conversion only contains operations legal for the target.

PiperOrigin-RevId: 258412243