bootstrap: Support `--` as an alternative to `--args` and `--test-args`

[jyn514]

Right now, x.py is pretty inconsistent with cargo's CLI. To pass arguments to compiletest, you use --test-args; to pass arguments to expand-yaml-anchors you use --args; to pass arguments to cargo you use CARGOFLAGS. I would like to unify these with the same syntax as cargo:

• x test ui -- --force-rerun --ignored would behave the same as x test ui --test-args --force-rerun --test-args --ignored today
• x run miri -- run --print=sysroot would behave the same as x run miri --args run --args --print=sysroot
• x fix -- --fix-broken would behave the same as CARGOFLAGS=--fix-broken x fix today, except that it would only pass it to cargo commands invoked by bootstrap, not when building bootstrap itself (avoiding the "argument --fix-broken not expected" error you'd get today).

[Teapot4195]

@rustbot claim

[Teapot4195]

Looking at the current setup for argument parsing, it appears that getopts::Options struct doesn't have a state engine based parser, so it wont be able to parse -- as a state changing flag. I'm thinking probably parse and translate the args in python then just pass it on to the bootstrap executable, (unless someone here has a better idea).

[jyn514]

@Teapot4195 let's not put any logic in python if we can help it, it's going away soon: #94829. I think it would be ok to switch to clap or another arg parser if it makes things easier.

[Teapot4195]

Ah, I was thinking something like getting rid of the python script was going to happen someday.
clap will probably do for the functionality we will need from it.

[Teapot4195]

I'm out of town for the rest of the week so if anyone would like to take this issue while I'm away, go for it.