Auto merge of #146165 - folkertdev:c-variadic-errors-take-2, r=lcnr

improve c-variadic error reporting

tracking issue: rust-lang/rust#44930

The parts of rust-lang/rust#143546 that don't require any particular knowledge about c-variadic functions.

This prepares the way for rejecting c-variadic functions that are also coroutines, safe functions, or associated functions.

Author: bors