[Runtime] Silence warnings on NULL checks of ObjC function addresses.

Some ObjC runtime calls are weak or strong depending on the deployment target. When strong, we get warnings that the NULL checks always succeed; silence them.

Some of the adjacent code looked up functions using dlsym when they aren't provided by the SDK. Our current minimum SDK always has them, so remove the dlsym workaround.

Author: mikeash

stdlib/public/runtime/Metadata.cpp

@@ -3029,13 +3029,6 @@ _swift_initClassMetadataImpl(ClassMetadata *self,
     setUpObjCRuntimeGetImageNameFromClass();
   }, nullptr);
 
-#ifndef OBJC_REALIZECLASSFROMSWIFT_DEFINED
-  // Temporary workaround until _objc_realizeClassFromSwift is in the SDK.
-  static auto _objc_realizeClassFromSwift =
-    (Class (*)(Class _Nullable, void *_Nullable))
-    dlsym(RTLD_NEXT, "_objc_realizeClassFromSwift");
-#endif
-
   // Temporary workaround until objc_loadClassref is in the SDK.
   static auto objc_loadClassref =
     (Class (*)(void *))
@@ -3078,7 +3071,7 @@ _swift_initClassMetadataImpl(ClassMetadata *self,
     // be safe to ignore the stub in this case.
     if (stub != nullptr &&
         objc_loadClassref != nullptr &&
-        _objc_realizeClassFromSwift != nullptr) {
+        &_objc_realizeClassFromSwift != nullptr) {
       _objc_realizeClassFromSwift((Class) self, const_cast<void *>(stub));
     } else {
       swift_instantiateObjCClass(self);
@@ -3129,14 +3122,7 @@ _swift_updateClassMetadataImpl(ClassMetadata *self,
                                const TypeLayout * const *fieldTypes,
                                size_t *fieldOffsets,
                                bool allowDependency) {
-#ifndef OBJC_REALIZECLASSFROMSWIFT_DEFINED
-  // Temporary workaround until _objc_realizeClassFromSwift is in the SDK.
-  static auto _objc_realizeClassFromSwift =
-    (Class (*)(Class _Nullable, void *_Nullable))
-    dlsym(RTLD_NEXT, "_objc_realizeClassFromSwift");
-#endif
-
-  bool requiresUpdate = (_objc_realizeClassFromSwift != nullptr);
+  bool requiresUpdate = (&_objc_realizeClassFromSwift != nullptr);
 
   // If we're on a newer runtime, we're going to be initializing the
   // field offset vector. Realize the superclass metadata first, even

stdlib/public/runtime/MetadataLookup.cpp

@@ -1948,7 +1948,7 @@ static void installGetClassHook() {
 
 #pragma clang diagnostic push
 #pragma clang diagnostic ignored "-Wunguarded-availability"
-  if (objc_setHook_getClass) {
+  if (&objc_setHook_getClass) {
     objc_setHook_getClass(getObjCClassByMangledName, &OldGetClassHook);
   }
 #pragma clang diagnostic pop