[Support] Make shouldReverseIterate constexpr (#156812)

This patch makes shouldReverseIterate constexpr, allowing compile-time
evaluation at call sites.

Author: kazutakahirata

llvm/include/llvm/Support/ReverseIteration.h

@@ -6,8 +6,7 @@
 
 namespace llvm {
 
-template<class T = void *>
-bool shouldReverseIterate() {
+template <class T = void *> constexpr bool shouldReverseIterate() {
 #if LLVM_ENABLE_REVERSE_ITERATION
   return detail::IsPointerLike<T>::value;
 #else