run-make-support: add `#[must_use]` to helpers where suitable

[jieyouxu]

For command wrappers like Rustc, they often have intermediate helper methods and actual "terminal" functions that executes the built command (e.g. run, run_fail, run_fail_assert_exit_code). Currently, it is easy to forget to call "terminal" functions.

For example,

rustc().input("foo.rs").arg("-Cprefer-dynamic");

does not actually execute and can unexpectedly silently pass. We should annotate intermediate helper methods with #[must_use] to ensure the built command is consumed, i.e. by executing the command.

[jieyouxu]

Maybe even consider drop bombs for command wrappers?