[clang] [clang] Add the candiscard attribute to suppress nodiscard (PR #154943)

[Erich Keane via cfe-commits]

https://github.com/erichkeane commented:

Would like some tests on templated type aliases 
(`template<typename T> using RetTy = SomeTempl::type`) to make sure that this properly suppresses, or collects the type.  Particularly when `RetTy`, or `::type` has either /both of the attributes (that is, the cartesian product of options for alias vs type).

https://github.com/llvm/llvm-project/pull/154943