Normally ImplSelfWitness would be replaced in an impl decl by the time
we are constructing the witness table. If it was part of an access to an
associated constant, rewrite constraint resolution would have replaced
that access with the value assigned to the associated constant already.
But we can use `.Self` in places where it converts and forms a
FacetValue, which preserves the ImplSelfWitness in the witness block of
the FacetValue. In that case, we want to replace it with the impl's
ImplWitness before we put it into the witness table.
This is a followup fix for
https://github.com/carbon-language/carbon-lang/pull/7930 which
introduces the CARBON_FATAL incorrectly, now that we know how to run
into it.
Add some tests that cover these situations, including one that tries to
access using the FacetValue and its witness, which fails, as a TODO
test.
Dana Jansens
e41862d55f