Turn cl::values() (for enum) from a vararg function to using C++ variadic template

Turn cl::values() (for enum) from a vararg function to using C++ variadic template

The core of the change is supposed to be NFC, however it also fixes
what I believe was an undefined behavior when calling:

 va_start(ValueArgs, Desc);

with Desc being a StringRef.

Differential Revision: https://reviews.llvm.org/D25342

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@283671 91177308-0d34-0410-b5e6-96231b3b80d8