C-variadic error improvements

[nicholasbishop]

A couple improvements for c-variadic errors:

1. Fix the bad-c-variadic error being emitted multiple times. If a function incorrectly contains multiple ... args, and is also not foreign or unsafe extern "C", only emit the latter error once rather than once per ....

2. Explicitly reject const C-variadic functions. Trying to use C-variadics in a const function would previously fail with an error like "destructor of VaListImpl<'_> cannot be evaluated at compile-time". Add an explicit check for const C-variadics to provide a clearer error: "functions cannot be both const and C-variadic". This also addresses one of the concerns in Tracking issue for RFC 2137: Support defining C-compatible variadic functions in Rust (c_variadic) #44930: "Ensure that even when this gets stabilized for regular functions, it is still rejected on const fn."

[rustbot]

r? @oli-obk

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[oli-obk]

lgtm. let me know about your opionion on making the diagnostic point to all the variadics at once

[oli-obk]

Thanks!

@bors r+ rollup

[bors]

📌 Commit f91b5ce has been approved by oli-obk

It is now in the queue for this repository.