[ADT] Modernize type traits in BitmaskEnum.h (NFC) (llvm#164567)

This patch replaces std::enable_if_t with std::void_t in two type
traits.  Both approaches enable the template specialization if and
only if the LLVM_BITMASK_LARGEST_ENUMERATOR enumerator exists.

Author: kazutakahirata

llvm/include/llvm/ADT/BitmaskEnum.h

@@ -106,15 +106,15 @@ struct is_bitmask_enum : std::false_type {};
 
 template <typename E>
 struct is_bitmask_enum<
-    E, std::enable_if_t<sizeof(E::LLVM_BITMASK_LARGEST_ENUMERATOR) >= 0>>
+    E, std::void_t<decltype(E::LLVM_BITMASK_LARGEST_ENUMERATOR)>>
     : std::true_type {};
 
 /// Trait class to determine bitmask enumeration largest bit.
 template <typename E, typename Enable = void> struct largest_bitmask_enum_bit;
 
 template <typename E>
 struct largest_bitmask_enum_bit<
-    E, std::enable_if_t<sizeof(E::LLVM_BITMASK_LARGEST_ENUMERATOR) >= 0>> {
+    E, std::void_t<decltype(E::LLVM_BITMASK_LARGEST_ENUMERATOR)>> {
   using UnderlyingTy = std::underlying_type_t<E>;
   static constexpr UnderlyingTy value =
       static_cast<UnderlyingTy>(E::LLVM_BITMASK_LARGEST_ENUMERATOR);