Add #[must_use] to stdin/stdout/stderr locks

[jkugelman]

Affected methods:

std::io           fn stdin_locked() -> StdinLock<'static>;
std::io::Stdin    fn lock(&self) -> StdinLock<'_>;
std::io::Stdin    fn into_locked(self) -> StdinLock<'static>;
std::io           fn stdout_locked() -> StdoutLock<'static>;
std::io::Stdout   fn lock(&self) -> StdoutLock<'_>;
std::io::Stdout   fn into_locked(self) -> StdoutLock<'static>;
std::io           fn stderr_locked() -> StderrLock<'static>;
std::io::Stderr   fn lock(&self) -> StderrLock<'_>;
std::io::Stderr   fn into_locked(self) -> StderrLock<'static>;

Parent issue: #89692

[rust-highfive]

r? @dtolnay

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[joshtriplett]

@bors r+

[bors]

📌 Commit e27bfb6 has been approved by joshtriplett