Replace ContextAwareDialectConversion fork with new upstream method

[j2kun]

llvm/llvm-project#140434

The idea is:

• For function declarations (no block arguments) use a custom pattern that ignores the type conversion system and manually converts types
• For everything else (a Value exists), define the type converter to take as input a Value

In both cases, we can still use the attribute attached to the op to store the data needed for the type conversion. So this should be a minimally invasive change.

[j2kun]

On further discussion with the maintainer, it seems the upstream PR will not suffice for our needs, and we'd need to wait for the full one-shot dialect conversion to be done, which may take quite a while.