Sema: Improved recovery from circular generic signature construction

[slavapestov]

Returning a null GenericSignature is not the right way to break a cycle,
because then callers have to be careful to handle the case of a null
GenericSignature together with a non-null GenericParamList, for example
in applyGenericArguments().

An even worse problem can occur when a GenericSignatureRequest for a
nested generic declaration requests the signature of the parent context,
which hits a cycle. In this case, we would build a signature where
the first generic parameter did not have depth 0.

This makes the requirement machine upset, so this patch implements a new
strategy to break such cycles. Instead of returning a null
GenericSignature, we build a signature with the correct generic
parameters, but no requirements. The generic parameters can be computed
just by traversing GenericParamLists, which does not trigger more
GenericSignatureRequests, so this should be safe.

[slavapestov]

@swift-ci Please smoke test