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

llvm-svn: 283671

llvm/lib/CodeGen/SafeStack.cpp

@@ -60,8 +60,7 @@ static cl::opt<UnsafeStackPtrStorageVal> USPStorage("safe-stack-usp-storage",
     cl::values(clEnumValN(ThreadLocalUSP, "thread-local",
                           "Thread-local storage"),
                clEnumValN(SingleThreadUSP, "single-thread",
-                          "Non-thread-local storage"),
-               clEnumValEnd));
+                          "Non-thread-local storage")));
 
 namespace llvm {