[Build] Use C++17 Constructor for tiktoken.cpp when C++20 is unavailable (#5025)

The basic_string_view constructor:
```
template< class It, class End >
constexpr basic_string_view( It first, End last );
```
requires C++20.

To allow the code to compile with C++17, use the basic_string_view constructor:
```
constexpr basic_string_view( const CharT* s, size_type count );
```

For #4661

Author: python3kgae

extension/llm/tokenizer/tiktoken.cpp

@@ -266,7 +266,11 @@ Tiktoken::_split_with_allowed_special_token(
     return std::make_pair(std::nullopt, input);
   }
 
+#if __cplusplus >= 202002L
   auto start = input.begin();
+#else
+  const char* start = input.data();
+#endif
   std::string special;
   while (true) {
     if (!re2::RE2::FindAndConsume(&input, *_special_token_regex, &special)) {
@@ -276,9 +280,15 @@ Tiktoken::_split_with_allowed_special_token(
 
     if (allowed_special.count(special) == 1) {
       // Found an allowed special token, split the text with it.
+#if __cplusplus >= 202002L
       return std::make_pair(
           special,
           re2::StringPiece(start, input.begin() - start - special.size()));
+#else
+      return std::make_pair(
+          special,
+          re2::StringPiece(start, (input.data() - start) - special.size()));
+#endif
     } // else try to find the next special token
   }