[lldb][DataFormatter] Fix format specifiers in LibCxxSliceArray summary provider (#85763)

This caused following warnings in an LLDB build:
```
[237/1072] Building CXX object tools/l...lusLanguage.dir/LibCxxSliceArray.cpp.o
/Volumes/Data/llvm-project/lldb/source/Plugins/Language/CPlusPlus/LibCxxSliceArray.cpp:38:53: warning: format specifies type 'unsigned long long' but the argument has type 'size_t' (aka 'unsigned long') [-Wformat]
   38 |   stream.Printf("stride=%" PRIu64 " size=%" PRIu64, stride, size);
      |                         ~~~~~~~~~                   ^~~~~~
/Volumes/Data/llvm-project/lldb/source/Plugins/Language/CPlusPlus/LibCxxSliceArray.cpp:38:61: warning: format specifies type 'unsigned long long' but the argument has type 'size_t' (aka 'unsigned long') [-Wformat]
   38 |   stream.Printf("stride=%" PRIu64 " size=%" PRIu64, stride, size);
      |                                          ~~~~~~~~~          ^~~~
2 warnings generated.
```

This patch simply changes the format specifiers to use the `%zu` for
`size_t`s.