Add char * overload for FlatBufferBuilder::CreateString() (#4583)

Without this change, the compiler tries to select the following overload
when CreateString is passed a `char *`:

    template<typename T>
    Offset<String> CreateString(const T &str) {
      return CreateString(str.c_str(), str.length());
    }

which is not valid since char pointers don't have methods.

(Fixes #4579)

Signed-off-by: Andrew Gunnerson <chenxiaolong@cxl.epac.to>