The asserts replaced by FLATBUFFERS_ASSERT. (#4701)

* The asserts replaced by FLATBUFFERS_ASSERT. Several asserts have converted to static_asserts.

* Regenerate header monster generate_code.sh

include/flatbuffers/idl.h

@@ -181,7 +181,7 @@ template<typename T> class SymbolTable {
       dict.erase(it);
       dict[newname] = obj;
     } else {
-      assert(false);
+      FLATBUFFERS_ASSERT(false);
     }
   }
 
@@ -485,7 +485,7 @@ class CheckedError {
     *this = other;  // Use assignment operator.
   }
 
-  ~CheckedError() { assert(has_been_checked_); }
+  ~CheckedError() { FLATBUFFERS_ASSERT(has_been_checked_); }
 
   bool Check() {
     has_been_checked_ = true;