Doing this when computing a canonical signature didn't really
make sense because canonical signatures are not canonicalized
any more strongly _with respect to the builder_; they just
canonicalize their requirement types.
Instead, let's do these checks after creating the signature in
computeGenericSignature().
The old behavior had another undesirable property; since the
canonicalization was done by registerGenericSignatureBuilder(),
we would always build a new GSB from scratch for every
signature we compute.
The new location also means we do these checks for protocol
requirement signatures as well. This flags an existing fixed
crasher where we still emit bogus same-type requirements in
the requirement signature, so I moved this test back into
an unfixed state.
