Rollup merge of #150436 - va-list-copy, r=workingjubilee,RalfJung

`c_variadic`: impl `va_copy` and `va_end` as Rust intrinsics

tracking issue: https://github.com/rust-lang/rust/issues/44930

Implement `va_copy` as (the rust equivalent of) `memcpy`, which is the behavior of all current LLVM targets. By providing our own implementation, we can guarantee its behavior. These guarantees are important for implementing c-variadics in e.g. const-eval.

Discussed in [#t-compiler/const-eval > c-variadics in const-eval](https://rust-lang.zulipchat.com/#narrow/channel/146212-t-compiler.2Fconst-eval/topic/c-variadics.20in.20const-eval/with/565509704).

I've also updated the comment for `Drop` a bit. The background here is that the C standard requires that `va_end` is used in the same function (and really, in the same scope) as the corresponding `va_start` or `va_copy`. That is because historically `va_start` would start a scope, which `va_end` would then close. e.g.

https://softwarepreservation.computerhistory.org/c_plus_plus/cfront/release_3.0.3/source/incl-master/proto-headers/stdarg.sol

```c
#define         va_start(ap, parmN)     {\
        va_buf  _va;\
        _vastart(ap = (va_list)_va, (char *)&parmN + sizeof parmN)
#define         va_end(ap)      }
#define         va_arg(ap, mode)        *((mode *)_vaarg(ap, sizeof (mode)))
```

The C standard still has to consider such implementations, but for Rust they are irrelevant. Hence we can use `Clone` for `va_copy` and `Drop` for `va_end`.

src/intrinsics/mod.rs

@@ -1506,7 +1506,7 @@ fn codegen_regular_intrinsic_call<'tcx>(
         }
 
         // FIXME implement variadics in cranelift
-        sym::va_copy | sym::va_arg | sym::va_end => {
+        sym::va_arg | sym::va_end => {
             fx.tcx.dcx().span_fatal(
                 source_info.span,
                 "Defining variadic functions is not yet supported by Cranelift",