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
diff --git a/llvm/tools/llvm-ar/llvm-ar.cpp b/llvm/tools/llvm-ar/llvm-ar.cpp
index 40e4a3a..b7cb959 100644
--- a/llvm/tools/llvm-ar/llvm-ar.cpp
+++ b/llvm/tools/llvm-ar/llvm-ar.cpp
@@ -93,7 +93,7 @@
     FormatOpt("format", cl::desc("Archive format to create"),
               cl::values(clEnumValN(Default, "default", "default"),
                          clEnumValN(GNU, "gnu", "gnu"),
-                         clEnumValN(BSD, "bsd", "bsd"), clEnumValEnd));
+                         clEnumValN(BSD, "bsd", "bsd")));
 
 static std::string Options;