c-variadic: improve implicit `&'reg VaList` argument diagnostics

cc #160491

see that PR for more detail and the FIXME(#160495) in the code, e.g.

https://github.com/rust-lang/rust/blob/c9ff496891c278ad660bc0ab85c1f0b72059464a/tests/ui/c-variadic/not-async.rs#L2-L14

results in

error: functions cannot be both `async` and C-variadic
  --> $DIR/not-async.rs:4:1
   |
LL | async unsafe extern "C" fn fn_cannot_be_async(x: isize, _: ...) {}
   | ^^^^^ `async` because of this                           ^^^^^^ C-variadic because of this

error: functions cannot be both `async` and C-variadic
  --> $DIR/not-async.rs:11:5
   |
LL |     async unsafe extern "C" fn method_cannot_be_async(x: isize, _: ...) {}
   |     ^^^^^ `async` because of this                               ^^^^^^ C-variadic because of this

error[E0700]: hidden type for `impl Future<Output = ()>` captures lifetime that does not appear in bounds
  --> $DIR/not-async.rs:4:65
   |
LL | async unsafe extern "C" fn fn_cannot_be_async(x: isize, _: ...) {}
   | ----------------------------------------------------------------^^
   | |                                                              |
   | |                                                              opaque type defined here
   | hidden type `{async fn body of fn_cannot_be_async()}` captures the anonymous lifetime as defined here

error[E0700]: hidden type for `impl Future<Output = ()>` captures lifetime that does not appear in bounds
  --> $DIR/not-async.rs:11:73
   |
LL |     async unsafe extern "C" fn method_cannot_be_async(x: isize, _: ...) {}
   |     --------------------------------------------------------------------^^
   |     |                                                                  |
   |     |                                                                  opaque type defined here
   |     hidden type `{async fn body of S::method_cannot_be_async()}` captures the anonymous lifetime as defined here

error: aborting due to 4 previous errors

with #160491 but should instead explicitly talk about the fact that we're capturing the &VaList here.