Auto merge of #99977 - BlackHoleFox:cfte-cstr, r=thomcc

Add validation to const fn CStr creation

Improves upon the existing validation that only worked when building the stdlib from source. `CStr::from_bytes_with_nul_unchecked` now utilizes `const_eval_select` to validate the safety requirements of the function when used as `const FOO: &CStr = CStr::from_bytes_with_nul_unchecked(b"Foobar\0");`.

This can help catch erroneous code written by accident and, assuming a new enough `rustc` in use, remove the need for boilerplate safety comments for this function in `const` contexts.

~~I think this might need a UI test, but I don't know where to put it. If this is a worth change, a perf run would be nice to make sure the `O(n)` validation isn't too bad. I didn't notice a difference building the stdlib tests locally.~~

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