[libc++] Fix erroneous internal capacity evaluation in vector<bool> (#120577)

This PR fixes the erroneous internal capacity evaluation in
`vector<bool>`, which caused a subsequent SIGSEGV error when calling
`flip()` on `vector<bool>`. By fixing the internal capacity evaluation,
the SIGSEGV is automatically resolved.

GitOrigin-RevId: 6c06253b8557102c52e252f9025ea3d393603324