Po-yao Chang b29301cd40
[libc++][format] Handle range-underlying-spec (#81914)
An immediate colon signifeis that the range-format-spec contains only
range-underlying-spec.

This patch allows this code to compile and run:
```c++
std::println("{::<<9?}", std::span<const char>{"Hello", sizeof "Hello"});
```
2024-03-04 08:05:01 +08:00
..
2024-02-05 14:31:16 -05:00