[libc++] Apply `[[nodiscard]]` to `in/out_ptr` (#167097)
...according to Coding Guidelines: `[[nodiscard]]` should be applied to
functions where discarding the return value is most likely a correctness
issue.
Changes to:
- [x] `inout_ptr()`
- [x] `out_ptr()`
At the time of impelentation the `[[nodiscard]]` policy has not been
established yet.
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Co-authored-by: Hristo Hristov <zingam@outlook.com>
