Fix clang conversion warnings in string_util::split_string_list (hsutter#1207)

* Fix clang conversion warnings in `string_util::split_string_list`

* Keeping fix while keeping signed type

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Co-authored-by: Herb Sutter <[email protected]>

Author: bluetarpmedia

include/cpp2util.h

@@ -389,7 +389,7 @@ auto split_string_list(std::string_view str)
         return std::isalnum(c) || c == '_';
     };
 
-    auto pos = 0;
+    auto pos = decltype(std::ssize(str)){ 0 };
     while( pos < std::ssize(str) ) {
         //  Skip non-alnum
         while (pos < std::ssize(str) && !is_id_char(str[pos])) {