[AST] Fix a warning

This patch fixes:

  clang/lib/AST/ByteCode/InterpBuiltin.cpp:767:12: error: unused
  variable 'PtrT' [-Werror,-Wunused-variable]

Author: kazutakahirata

clang/lib/AST/ByteCode/InterpBuiltin.cpp

@@ -768,6 +768,7 @@ static bool interp__builtin_addressof(InterpState &S, CodePtr OpPC,
       S.getContext().classify(Call->getArg(0)).value_or(PT_Ptr);
   assert(PtrT == PT_Ptr &&
          "Unsupported pointer type passed to __builtin_addressof()");
+  (void)PtrT;
   return true;
 }