[clang-tidy] checkOverridingFunctionReturnType - use castAs<> instead of getAs<> to avoid dereference of nullptr

The pointers are referenced immediately, so assert the cast is correct instead of returning nullptr

clang-tools-extra/clang-tidy/bugprone/VirtualNearMissCheck.cpp

@@ -40,11 +40,11 @@ static bool checkOverridingFunctionReturnType(const ASTContext *Context,
                                               const CXXMethodDecl *BaseMD,
                                               const CXXMethodDecl *DerivedMD) {
   QualType BaseReturnTy = BaseMD->getType()
-                              ->getAs<FunctionType>()
+                              ->castAs<FunctionType>()
                               ->getReturnType()
                               .getCanonicalType();
   QualType DerivedReturnTy = DerivedMD->getType()
-                                 ->getAs<FunctionType>()
+                                 ->castAs<FunctionType>()
                                  ->getReturnType()
                                  .getCanonicalType();