Extensions cannot capture generic parameters from outer contexts.
Their generic parameter list does not point to the outer
parameter list, the signature does not contain outer parameters
and they do not appear in the extension's generic environment.
Furthermore, the depth/index of the extension's parameters may
clash with outer parameters, which would break all kinds of
invariants.
This patch changes these DeclContext methods to return false or
nullptr even if an extension is contained in a generic context:
- isGenericContext()
- getGenericParamsOfContext()
- getGenericSignatureOfContext()
- getGenericEnvironmentOfContext()
