IRGen: Generalize the nullable optimization for single-payload enums.

Augment the `isSingleRetainablePointer` check that allows IRGen to avoid adding branching around retain/release operations on enums that use the null pointer extra inhabitant with a more general "can value witness extra inhabitants" method on TypeInfo, which says whether a type's retain/release operations are safe to invoke on some or all of its extra inhabitants. This lets us generalize the optimization to include things like `String?` or `ClassProtocol?` which are common types with a nullable pointer in them.

lib/IRGen/GenStruct.cpp

@@ -219,7 +219,7 @@ namespace {
         return false;
       return fields[0].getTypeInfo().isSingleRetainablePointer(expansion, rc);
     }
-    
+       
     void verify(IRGenTypeVerifierFunction &IGF,
                 llvm::Value *metadata,
                 SILType structType) const override {