[roll] Roll fuchsia [stdcompat] Avoid implicit conversion, negative shift in bit_floor

std::bit_floor<T> returns T but it's built out of std::bit_width<T>
which returns int. The implementation implicitly converted bit_width's
int return value to T which can (depending on T) result in sign or width
conversion errors.

By definition this is safe becuase bit_width returns the number of bits
needed to store a number which will always fit in the type of that
number.

This also handles the case where the value is 0.

And this updates some copy & paste typos in the test that have no impact
other than confusing failure messages.

Original-Bug: b/259746255
Original-Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/855246
Original-Revision: 2c44bd8ba2ac39a79af11bf30770b5ab5bc65b40
GitOrigin-RevId: 7d16a8b26be69899366259265b39fdd4ac862f8e
Change-Id: I617636838a498903ec39be829a4ac4b1b0156d35