Lift Requirement and Parameter Accessors up to GenericSignature

Start treating the null {Can}GenericSignature as a regular signature
with no requirements and no parameters. This not only makes for a much
safer abstraction, but allows us to simplify a lot of the clients of
GenericSignature that would previously have to check for null before
using the abstraction.

lib/Sema/CSStep.cpp

@@ -583,7 +583,7 @@ bool IsDeclRefinementOfRequest::evaluate(Evaluator &evaluator,
 
   auto result = TypeChecker::checkGenericArguments(
       declA->getDeclContext()->getParentModule(),
-      genericSignatureB->getRequirements(),
+      genericSignatureB.getRequirements(),
       QueryTypeSubstitutionMap{ substMap });
 
   if (result != RequirementCheckResult::Success)