[libc++] Simplify vector<bool> fill constructors (#160521)

#119632 introduced a code size and performance regression. This was
verified by running the included benchmark against trunk and trunk with
#119632 reverted. Instead of actually reverting that patch, we can
inline `__construct_at_end` into the few places it's used instead,
simplifying the implementation further (by not handling special cases we
never actually encounter).

```
Benchmark                   Baseline    Candidate    Difference    % Difference
------------------------  ----------  -----------  ------------  --------------
BM_vector_bool_size_ctor       29.91         8.56        -21.35          -71.37
```

0 files changed, 0 insertions, 0 deletions