ea85018ccf
Because of the way that the `pass` subcommand orders its arguments, the
positional "single-file" input cannot be optional with a default value, because
it is followed by required positional arguments. If it were optional, that would
result in argument ambiguity where `clap` cannot tell if the optional positional
argument is supplied, or if the given argument is the next required positional
argument.
Before this commit:
```
$ cargo run --bin clif-util -- pass ./filetests/dce/basic.clif dce
Compiling cranelift-tools v0.21.0 (file:///Users/fitzgen/src/cranelift)
Finished dev [unoptimized + debuginfo] target(s) in 4.38s
Running `target/debug/clif-util pass ./filetests/dce/basic.clif dce`
thread 'main' panicked at 'Found positional argument which is not required with a lower index than a required positional argument: "single-file" index 1', /Users/fitzgen/.cargo/registry/src/github.com-1ecc6299db9ec823/clap-2.32.0/src/app/parser.rs:612:21
note: Run with `RUST_BACKTRACE=1` for a backtrace.
```
After this commit:
```
$ cargo run --bin clif-util -- pass ./filetests/dce/basic.clif dce
Compiling cranelift-filetests v0.21.0 (file:///Users/fitzgen/src/cranelift/lib/filetests)
Compiling cranelift-tools v0.21.0 (file:///Users/fitzgen/src/cranelift)
Finished dev [unoptimized + debuginfo] target(s) in 5.96s
Running `target/debug/clif-util pass ./filetests/dce/basic.clif dce`
1 tests
```