[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.

include/swift/SIL/SILFunction.h

@@ -71,7 +71,7 @@ public:
   };
 
   static SILSpecializeAttr *create(SILModule &M,
-                                   GenericSignature *specializedSignature,
+                                   GenericSignature specializedSignature,
                                    bool exported, SpecializationKind kind);
 
   bool isExported() const {
@@ -90,7 +90,7 @@ public:
     return kind;
   }
 
-  GenericSignature *getSpecializedSignature() const {
+  GenericSignature getSpecializedSignature() const {
     return specializedSignature;
   }
 
@@ -103,11 +103,11 @@ public:
 private:
   SpecializationKind kind;
   bool exported;
-  GenericSignature *specializedSignature;
+  GenericSignature specializedSignature;
   SILFunction *F = nullptr;
 
   SILSpecializeAttr(bool exported, SpecializationKind kind,
-                    GenericSignature *specializedSignature);
+                    GenericSignature specializedSignature);
 };
 
 /// SILFunction - A function body that has been lowered to SIL. This consists of