Fix some clippy lints for library/std/src/sys/windows
These issues were shown by running `x clippy` on `library/std` and filtering for `windows/` paths. I think running clippy on the full std would be great but I wanted to start smaller and with something that's hopefully easier to review. It'd be good to eventually run clippy in CI but that's a bigger conversation.
I've created separate commits for each clippy lint fixed (with the commit title set to the lint name) and reviewed the changes myself. Most of the fixes here are trivial.
r? libs
c387f012b1