StringRef.h: __builtin_strlen seems to exist in VS 2017 MSVC 19.16 or later

This is a follow-up to ff837aa, as discussed on the llvm-commits
thread for that one.

(cherry picked from commit 1b3d166)

Author: zmodem

llvm/include/llvm/ADT/StringRef.h

@@ -78,7 +78,7 @@ namespace llvm {
 #if __cplusplus > 201402L
       return std::char_traits<char>::length(Str);
 #elif __has_builtin(__builtin_strlen) || defined(__GNUC__) || \
-    (defined(_MSC_VER) && _MSC_VER >= 1920)
+    (defined(_MSC_VER) && _MSC_VER >= 1916)
       return __builtin_strlen(Str);
 #else
       const char *Begin = Str;