Add --force-defaults option to flatc [C++, parser] (#4729)

* Add --force-defaults option to flatc

To emit default values for fields which are not present or which are set
to the default value.

* flatc option --force-defaults should have a default value (false) and take action on the builder_ within the Parser constructor

* Add help text from flatc --force-defaults to Compiler.md doc

* Clarified docs for flatc --force-defaults, and imply that this behaviour is not normally needed.

* Updated docs and flatc help text for --force-defaults option

include/flatbuffers/idl.h

@@ -392,6 +392,7 @@ struct IDLOptions {
   bool protobuf_ascii_alike;
   bool size_prefixed;
   std::string root_type;
+  bool force_defaults;
 
   // Possible options for the more general generator below.
   enum Language {
@@ -452,6 +453,7 @@ struct IDLOptions {
         reexport_ts_modules(true),
         protobuf_ascii_alike(false),
         size_prefixed(false),
+        force_defaults(false),
         lang(IDLOptions::kJava),
         mini_reflect(IDLOptions::kNone),
         lang_to_generate(0) {}
@@ -527,6 +529,9 @@ class Parser : public ParserState {
         source_(nullptr),
         anonymous_counter(0),
         recurse_protection_counter(0) {
+    if (opts.force_defaults) {
+      builder_.ForceDefaults(true);
+    }
     // Start out with the empty namespace being current.
     empty_namespace_ = new Namespace();
     namespaces_.push_back(empty_namespace_);