[llvm] [LangRef] allow omitting `va_end` (PR #203087)

[Eli Friedman via llvm-commits]

https://github.com/efriedma-quic commented:

https://groups.google.com/g/comp.std.c/c/cld4zwoKaFw indicates that some very old compilers might have needed va_end, but I don't know of any specific ABI that needs it.  It's probably fine to loosen the rules like this.

We probably want to mention somewhere that the underlying argument list is destroyed when the function returns.

We probably also want to modify the description of va_copy to make it clear that it's okay to just memcpy on targets where va_list is trivially copyable.

https://github.com/llvm/llvm-project/pull/203087