rustc bad cli arg error messages doesn't respect `--error-format=json`

Not surprising (and I don't really consider this a bug necessarily), but something like

$ rustc --error-format=json --emit
error: Argument to option 'emit' missing
       Usage:
           --emit [asm|llvm-bc|llvm-ir|obj|metadata|link|dep-info|mir]
                               Comma separated list of types of output for the
                               compiler to emit

doesn't respect --error-format=json. I think that is entirely reasonable, since --error-format itself might be missing its value or get passed some invalid value 🤷

Just opening an issue to note this, because the implication for ui tests is that this obviously won't get picked up by compiletest for the //~? error annotations which are based on JSON diagnostic messages.

For test writers, the implication is simply that you can't use

//@ compile-flags: --emit

//~? ERROR: Argument to option 'emit' missing

and you'll need to use

//@ compile-flags: --emit
//@ error-pattern: Argument to option 'emit' missing

as an fallback.

Start by reproducing the rustc --error-format=json --emit behavior and read the compiletest UI-test handling for //~? JSON diagnostics and //@ error-pattern. The issue does not define a requested change, so first establish whether malformed CLI argument errors should be represented in JSON and what tests should verify the chosen behavior.