This catches another case where resolveType() could cause infinite
recursion. No test case, but this prevents crashers from regressing
with a subsequent patch.
This would just set the NominalTypeDecl's declared type to
ErrorType, which caused problems elsewhere.
Instead, generalize the logic used for AbstractFunctionDecl.
This correctly wires up the GenericTypeParamDecl's archetypes even
if the signature didn't validate, fixing crashes if the generic
parameters of the type are referenced.
