[ADT] Mark `llvm::array_lengthof` as deprecated

As a follow-up of 5e96cea1db, mark
`llvm::array_lengthof` as deprecated in favor of using `std::size` function
directly.

Differential Revision: https://reviews.llvm.org/D133502

llvm/include/llvm/ADT/STLArrayExtras.h

@@ -18,6 +18,8 @@
 
 #include <cstddef>
 
+#include "llvm/Support/Compiler.h"
+
 namespace llvm {
 
 //===----------------------------------------------------------------------===//
@@ -26,7 +28,8 @@ namespace llvm {
 
 /// Find the length of an array.
 template <class T, std::size_t N>
-constexpr inline size_t array_lengthof(T (&)[N]) {
+constexpr inline LLVM_DEPRECATED("Use std::size instead.", "std::size") size_t
+    array_lengthof(T (&)[N]) {
   return N;
 }