[ADT] Replace Min with variadic std::min

Without this patch, NumTagBits and MinTag use a recursive template Min
to compute Min.

This patch replaces Min with variadic std::min.

While I'm at it, this patch changes the type of NumTagBits to static
constexpr int.

Author: kazutakahirata

llvm/include/llvm/ADT/PointerSumType.h

@@ -9,9 +9,10 @@
 #ifndef LLVM_ADT_POINTERSUMTYPE_H
 #define LLVM_ADT_POINTERSUMTYPE_H
 
-#include "llvm/ADT/bit.h"
 #include "llvm/ADT/DenseMapInfo.h"
+#include "llvm/ADT/bit.h"
 #include "llvm/Support/PointerLikeTypeTraits.h"
+#include <algorithm>
 #include <cassert>
 #include <cstdint>
 #include <type_traits>
@@ -226,19 +227,13 @@ struct PointerSumTypeHelper : MemberTs... {
   };
 
   // Next we need to compute the number of bits available for the discriminant
-  // by taking the min of the bits available for each member. Much of this
-  // would be amazingly easier with good constexpr support.
-  template <uintptr_t V, uintptr_t... Vs>
-  struct Min : std::integral_constant<
-                   uintptr_t, (V < Min<Vs...>::value ? V : Min<Vs...>::value)> {
-  };
-  template <uintptr_t V>
-  struct Min<V> : std::integral_constant<uintptr_t, V> {};
-  enum { NumTagBits = Min<MemberTs::TraitsT::NumLowBitsAvailable...>::value };
+  // by taking the min of the bits available for each member.
+  static constexpr int NumTagBits =
+      std::min({MemberTs::TraitsT::NumLowBitsAvailable...});
 
   // Also compute the smallest discriminant and various masks for convenience.
   constexpr static TagT MinTag =
-      static_cast<TagT>(Min<MemberTs::Tag...>::value);
+      static_cast<TagT>(std::min({static_cast<TagT>(MemberTs::Tag)...}));
   enum : uint64_t {
     PointerMask = static_cast<uint64_t>(-1) << NumTagBits,
     TagMask = ~PointerMask