libc++ currently has very limited test coverage for `std::ranges{fill, fill_n, find}`
with `vector<bool>::iterator` optimizations. Specifically, the existing tests for
`std::ranges::fill` only covers cases of 1 - 2 bytes, which is merely 1/8 to 1/4
of the `__storage_type` word size. This renders the tests insufficient to validate
functionality for whole words, with or without partial words (which necessitates at
least 8 bytes of data). Moreover, no tests were provided for `ranges::{find, fill_n}`
with `vector<bool>::iterator` optimizations. This PR fills in the gap.
This PR addresses an undefined behavior that arises when using the
`std::fill` and `std::fill_n` algorithms, as well as their ranges
counterparts `ranges::fill` and `ranges::fill_n`, with `vector<bool, Alloc>`
that utilizes a custom-sized allocator with small integral types.
When we ship LLVM 16, <ranges> won't be considered experimental anymore.
We might as well do this sooner rather than later.
Differential Revision: https://reviews.llvm.org/D132151
