[NFC] Adopt TypeBase-isms for GenericSignature

Structurally prevent a number of common anti-patterns involving generic
signatures by separating the interface into GenericSignature and the
implementation into GenericSignatureBase.  In particular, this allows
the comparison operators to be deleted which forces callers to
canonicalize the signature or ask to compare pointers explicitly.

lib/IRGen/GenMeta.h

@@ -157,7 +157,7 @@ namespace irgen {
   GenericRequirementsMetadata addGenericRequirements(
                                           IRGenModule &IGM,
                                           ConstantStructBuilder &B,
-                                          GenericSignature *sig,
+                                          GenericSignature sig,
                                           ArrayRef<Requirement> requirements);
 
 } // end namespace irgen