Don't look for a user-defined conversion (implementation of `As` or
`ImplicitAs`) if the builtin conversion to a facet type fails impl
lookup. This is the behavior we want, and reduces noise in diagnostics.
Partial implementation of #5122. Still to do:
* Give an error if the users tries to implement such a conversion, since
it is now unreachable.
* Add notes to the diagnostic explaining why impl lookup failed.
---------
Co-authored-by: Josh L <josh11b@users.noreply.github.com>