[C++] Add Command-Line Flag to Suppress MIN and MAX Enums (#7705)

Add the --no-minmax-values flag to prevent flatc from generating C++
enums with MIN and MAX enumerated values that otherwise would be set
to the inclusive lower and upper bound respectively of the enum.

This command-line flag is needed to avoid collisions when an enum that
is being ported to FlatBuffers already has a MIN or MAX enumerated
value.

It is also needed to work around a long-standing problem with
magic_enum that causes magic_enum to not see enumerated values that
are not unique.  For example, if FlatBuffers sets MIN = FOO and MAX =
BAR, MIN and FOO share the same underlying value so they are not
unique.  The same is true of MAX and BAR.  This prevents magic_enum
from converting FOO and BAR to and from strings as well as causing
magic_enum to return a count of enumerated values that is two fewer
than it should be.

Co-authored-by: Paul Serice <paul@serice.net>

src/idl_gen_cpp.cpp

@@ -1223,6 +1223,8 @@ class CppGenerator : public BaseGenerator {
       FLATBUFFERS_ASSERT(minv && maxv);
 
       code_.SetValue("SEP", ",\n");
+
+      // MIN & MAX are useless for bit_flags
       if (enum_def.attributes.Lookup("bit_flags")) {
         code_.SetValue("KEY", GenEnumValDecl(enum_def, "NONE"));
         code_.SetValue("VALUE", "0");
@@ -1233,7 +1235,7 @@ class CppGenerator : public BaseGenerator {
                        NumToStringCpp(enum_def.AllFlags(),
                                       enum_def.underlying_type.base_type));
         code_ += "{{SEP}}  {{KEY}} = {{VALUE}}\\";
-      } else {  // MIN & MAX are useless for bit_flags
+      } else if (opts_.emit_min_max_enum_values) {
         code_.SetValue("KEY", GenEnumValDecl(enum_def, "MIN"));
         code_.SetValue("VALUE", GenEnumValDecl(enum_def, Name(*minv)));
         code_ += "{{SEP}}  {{KEY}} = {{VALUE}}\\";