libstdc++: Cast -1 to size_t in <format> [PR119429]

This avoids a runtime error from Clang's annoying -fsanitize=integer
(even though it's not undefined and behaves correctly).

libstdc++-v3/ChangeLog:

	PR libstdc++/119429
	* include/std/format (__format::_Scanner::_Scanner): Cast
	default argument to size_t.

libstdc++-v3/include/std/format

@@ -4051,7 +4051,7 @@ namespace __format
       } _M_pc;
 
       constexpr explicit
-      _Scanner(basic_string_view<_CharT> __str, size_t __nargs = -1)
+      _Scanner(basic_string_view<_CharT> __str, size_t __nargs = (size_t)-1)
       : _M_pc(__str, __nargs)
       { }