If 'extend impl' is invalid, mark the impl as invalid by putting an
ErrorInst in the witness_id field.
The construction of the witness_id can otherwise return an ErrorInst but
impl lookup was not checking for that. Now have impl lookup check for an
error there before attempting to deduce generic parameters, which avoids
infinite recursion through deduction in cases like a cyclical impl of
itself.
Adds a testcase for infinite impl-of-itself lookup found by fuzzer.
Dana Jansens
d6ce8f192d