Add --warnings-as-errors to flatc compiler. (#7034)

* Add --warnings-as-errors to flatc compiler.

With this option set, flatc will return an error on parsing if
any warnings occurred.

* Add unit test for opts.warnings_as_errors.

* Change explicit option setting to default.

include/flatbuffers/idl.h

@@ -591,6 +591,7 @@ struct IDLOptions {
   std::string filename_suffix;
   std::string filename_extension;
   bool no_warnings;
+  bool warnings_as_errors;
   std::string project_root;
   bool cs_global_alias;
   bool json_nested_flatbuffers;
@@ -681,6 +682,7 @@ struct IDLOptions {
         filename_suffix("_generated"),
         filename_extension(),
         no_warnings(false),
+        warnings_as_errors(false),
         project_root(""),
         cs_global_alias(false),
         json_nested_flatbuffers(true),
@@ -787,6 +789,7 @@ class Parser : public ParserState {
         root_struct_def_(nullptr),
         opts(options),
         uses_flexbuffers_(false),
+        has_warning_(false),
         advanced_features_(0),
         source_(nullptr),
         anonymous_counter_(0),
@@ -1021,6 +1024,7 @@ class Parser : public ParserState {
 
   IDLOptions opts;
   bool uses_flexbuffers_;
+  bool has_warning_;
 
   uint64_t advanced_features_;