Fix "pointer is null" static analyzer warnings. NFCI.

Use castAs<> instead of getAs<> since the pointers are dereferenced immediately and castAs will perform the null assertion for us.
diff --git a/clang/lib/Sema/SemaCodeComplete.cpp b/clang/lib/Sema/SemaCodeComplete.cpp
index 0f2434f..e14c63d 100644
--- a/clang/lib/Sema/SemaCodeComplete.cpp
+++ b/clang/lib/Sema/SemaCodeComplete.cpp
@@ -4806,7 +4806,7 @@
       }
 
       // Add properties from the protocols in a qualified interface.
-      for (auto *I : BaseType->getAs<ObjCObjectPointerType>()->quals())
+      for (auto *I : BaseType->castAs<ObjCObjectPointerType>()->quals())
         AddObjCProperties(CCContext, I, true, /*AllowNullaryMethods=*/true,
                           CurContext, AddedProperties, Results,
                           IsBaseExprStatement, /*IsClassProperty*/ false,
@@ -4819,7 +4819,7 @@
               BaseType->getAs<ObjCObjectPointerType>())
         Class = ObjCPtr->getInterfaceDecl();
       else
-        Class = BaseType->getAs<ObjCObjectType>()->getInterface();
+        Class = BaseType->castAs<ObjCObjectType>()->getInterface();
 
       // Add all ivars from this class and its superclasses.
       if (Class) {
@@ -7743,8 +7743,8 @@
   if (IsInstanceMethod &&
       (ReturnType.isNull() ||
        (ReturnType->isObjCObjectPointerType() &&
-        ReturnType->getAs<ObjCObjectPointerType>()->getInterfaceDecl() &&
-        ReturnType->getAs<ObjCObjectPointerType>()
+        ReturnType->castAs<ObjCObjectPointerType>()->getInterfaceDecl() &&
+        ReturnType->castAs<ObjCObjectPointerType>()
                 ->getInterfaceDecl()
                 ->getName() == "NSArray"))) {
     std::string SelectorName = (Twine(Property->getName()) + "AtIndexes").str();
@@ -8130,8 +8130,8 @@
   if (!IsInstanceMethod &&
       (ReturnType.isNull() ||
        (ReturnType->isObjCObjectPointerType() &&
-        ReturnType->getAs<ObjCObjectPointerType>()->getInterfaceDecl() &&
-        ReturnType->getAs<ObjCObjectPointerType>()
+        ReturnType->castAs<ObjCObjectPointerType>()->getInterfaceDecl() &&
+        ReturnType->castAs<ObjCObjectPointerType>()
                 ->getInterfaceDecl()
                 ->getName() == "NSSet"))) {
     std::string SelectorName =