Retire llvm::array_endof in favor of non-member std::end.

While there make array_lengthof constexpr if we have support for it.

llvm-svn: 206112
diff --git a/llvm/unittests/ADT/StringRefTest.cpp b/llvm/unittests/ADT/StringRefTest.cpp
index c7fd9d0..d80179b 100644
--- a/llvm/unittests/ADT/StringRefTest.cpp
+++ b/llvm/unittests/ADT/StringRefTest.cpp
@@ -9,6 +9,7 @@
 
 #include "llvm/ADT/StringRef.h"
 #include "llvm/ADT/Hashing.h"
+#include "llvm/ADT/STLExtras.h"
 #include "llvm/ADT/SmallVector.h"
 #include "llvm/ADT/StringExtras.h"
 #include "llvm/Support/Allocator.h"