[APInt] Replace enum with static constexpr member variables. NFC

With C++17 we no longer need the enum to prevent ODR use.

Author: topperc

llvm/include/llvm/ADT/APInt.h

@@ -79,13 +79,11 @@ class [[nodiscard]] APInt {
 public:
   typedef uint64_t WordType;
 
-  /// This enum is used to hold the constants we needed for APInt.
-  enum : unsigned {
-    /// Byte size of a word.
-    APINT_WORD_SIZE = sizeof(WordType),
-    /// Bits in a word.
-    APINT_BITS_PER_WORD = APINT_WORD_SIZE * CHAR_BIT
-  };
+  /// Byte size of a word.
+  static constexpr unsigned APINT_WORD_SIZE = sizeof(WordType);
+
+  /// Bits in a word.
+  static constexpr unsigned APINT_BITS_PER_WORD = APINT_WORD_SIZE * CHAR_BIT;
 
   enum class Rounding {
     DOWN,