[hip] Allow the declaration of functions with variadic arguments in HIP.

Summary:
- As variadic parameters have the lowest rank in overload resolution,
  without real usage of `va_arg`, they are commonly used as the
  catch-all fallbacks in SFINAE. As the front-end still reports errors
  on calls to `va_arg`, the declaration of functions with variadic
  arguments should be allowed in general.

Reviewers: jlebar, tra, yaxunl

Subscribers: cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D69389