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/Serialization/Deserialization.cpp

@@ -1816,7 +1816,7 @@ giveUpFastPath:
       }
 
       bool found = false;
-      for (auto paramTy : currentSig->getGenericParams()) {
+      for (auto paramTy : currentSig.getGenericParams()) {
         if (paramTy->getIndex() == paramIndex &&
             paramTy->getDepth() == depth) {
           values.clear();
@@ -4110,7 +4110,7 @@ public:
         paramCount += paramList->size();
       }
       assert(paramCount ==
-             extension->getGenericSignature()->getGenericParams().size());
+             extension->getGenericSignature().getGenericParams().size());
     }
 #endif