The current implementation is overly conservative and disable all
possible caching as soon as a context-aware conversion is present.
However the context-aware conversion only affects subsequent converters,
we can cache the previous ones.
This isn't NFC because if fixed a bug where we use to unconditionally
cache when using the `convertType(Type t, ...` API, while now all APIs
are aware of context-aware conversions.
b22f94dcc5