c-variadic: allow c-variadic inherent and trait methods
tracking issue: https://github.com/rust-lang/rust/issues/44930
Continuing the work of https://github.com/rust-lang/rust/pull/146342, allow inherent and trait methods to be c-variadic. However, a trait that contains a c-variadic method is no longer dyn-compatible.
There is, presumably, some way to make c-variadic methods dyn-compatible. However currently, we don't have confidence that it'll work reliably: when methods from a `dyn` object are cast to a function pointer, a `ReifyShim` is created. If that shim is c-variadic, it would need to forward the C variable argument list.
That does appear to work, because the `va_list` is not represented in MIR at all in this case, so the registers from the call site are untouched by the shim and can be read by the actual implementation. That just does not seem like a solid implementation.
Also, intuitively, why would c-variadic function, primarily needed for FFI, need to be used with `dyn` objects at all? We can revisit this limitation if a need arises.
r? `@workingjubilee`
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